Okay, guess I should wade in here.
As some people may know, I usually do a LOT of costuming events- we do a lot of social and charity work as well as our own 'for fun' ones. These range anywhere from your usual big cons, to smaller fairs, family fun days, visits to hospitals/ care centres, attending festivals, attending in store events/ game/ movie releases, fundraising events, charity run events for their members (usually children and their families), May the 4th be with you events, or even small scale charity stuff like making videos to inspire a sick kid, all the way to just meeting up for the fun of it- there's a local event called NekoNation which is a combo dance party/ anime con at night club, and another one called Cosplay Chills which is a monthly meet up on cosplayers in a local park. A video gaming bar also holds semi regular events and they run the con afterparties as well. There have been periods where I'll do multiple events in one week (like just before my op we did 4 Halo Spartan events in 9 days) and other times we will go weeks without anything. There are a few things I have picked up and learnt along the way. I have also helped organise the costumers for an event run by my cousin, and as second in charge (hubby is in charge) of our local 405th Halo costumers squad we are becoming more experienced in organising ourselves. A lot of what I say will come from a more group orientated perspective but still applies if you are on your own or with a few friends.
Handlers: they are definitely the unsung heros. People often don’t give them enough credit, sometimes look down on them because they aren’t in costume or just disregard them. Handlers not only can be the packhorse and look after your gear- for us at cons, this usually means my sister with water bottles for each of us- but MOST IMPORTANTLY they are your contact with the public. When you are in full armour, have your face covered by a helmet or mask, in a mobility restricting outfit etc I cannot stress their importance enough. They act as your protection against idiots and when you have people crowding around for photos can help keep people in order or even take the pictures. Sadly at cons and any kind of public event you will always get the idiots who decide to hit you on the head, poke at your costume, steal your props and weapons or even full on body tackle you. It’s shocking behaviour but some idiots think they can do whatever the hell they want and you have to be prepared for this. They are also your physical guide if you have limited visibility. If your visibility or mobility are severely impaired by your costume consider having agreed upon signals/ phrases so if your friend is madly waving at you to do something you know what they are meaning!
Being a handler DOESN’T mean you can’t be in costume yourself, you just make it a fully mobile soft costume. For example hubby and I have a huge costume in the planning- for Gundam fans out there it is a unicorn gundam, which will include platforms/ leg stilts to get more height and my ambitious husband wants to make it so that he can hit a switch and it will transform from its unicorn mode to destroy mode. From experience doing events with the transformer costumers this means he isn’t going to be moving far- costumes like that you can only shuffle in and the space you need around you means you are restricted to just inside the entrance or outside in the lobby. You also DEFINITELY need a handler or two or three just to GUIDE the person! As a result I’ll be making a gundam pilot suit (probably from Double O Gundam as that is my fav and will be easier and cooler than the bulky Unicorn Gundam pilot suits!) so that I am still in costume and in universe but my entire role will be looking after hubby.
Take Spares: what do I mean? Take spare batteries, safety pins, stick on Velcro, pieces of foam padding, hell even a hot glue gun and needle and thread! Believe me it REALLY sucks when your lightsaber runs out of juice and you have no spare batteries! Take more than you think you will need, because you will thank me later for it! Also take some basic lightsaber tools with you- whatever sized hex keys/ wrenches you need for your particular saber. This way if your blade gets loose you can fix it, or you can swap it out. For larger events where we costumers have our own changing room or space at a booth I bring my full lightsaber kit which includes my spare parts, tools, batteries, etc. I have regularly helped others with their lightsabers or provided spare batteries- or even whole lightsabers when their hilts have run into a problem and stopped working and they need a saber! (other people's hilts feel very weird hanging off my belt however!)
You will almost definitely have something go wrong on your costume especially if it is only the first or second time you’ve taken it out. Things will come loose or fall off, or slip down! A lot of costumers will always make sure they have a hot glue gun, bottle of super glue, spare Velcro and often spare foam as well as the all important safety pins! It doesn’t have to take up much space, we bought a tiny hot glue gun on top of our really exxy one for just this purpose, and can be left in your car or a check in bag. Also keep your eyes out for the Fix it superheros (like Captain Patch-it) that get around at most of these cons, they might be willing to lend you a hand
Have a check in bag: most cons will allow you to check in your weapons or bags. This can be extremely handy and it usually doesn’t cost much. You can leave a supply of bits to fix your costumes, and food and water, and have somewhere to safely leave your shopping so you don’t have to carry it- and inevitably either get stuck posing for pictures holding it or risk putting it down- we have come up with some inventive poses to help ‘guard’ or hide our gear while doing this!
Incorporate pouches into your outfit: either build hidden pouches/ pockets into your costume (often Halo costumers will build pouches into the inside of their thighs, or bracers, or torso armour) or have pouches or pockets as part of your costume. When at cons I don a utility belt with single back pouch for my Spartan and my Jedi has three pouches, each with their own purpose- one holds my cosplay business cards, pommels and my toolkit, one holds batteries, and the last big pouch holds batteries, asthma puffer, wallet, phone, extra batteries etc. I can be almost completely self sufficient as a result. Some people have a small pouch on a lanyard around their neck which just holds their phone, ID and some money and goes inside their costume/ armour. If you have gloves or bracers test beforehand that you can get into your pouches too! Leading on from this you can also incorporate water containers into your costume- either have a canteen in a pouch, or a water bottle designed/ painted to look like part of the costume- our Ghost Busters have water bottles that look like the slime containers

Having a camelbak in armour can be a valid tactic- and so long as you have it mounted right it won’t get overly warm or weigh you down weirdly.
Cooling: remember to keep cool! This is part of costume design as well, but try not to make your costume overly hot if you can avoid it. Have spaces where air can flow and use materials that are cooler- natural fibres and compression wear/ active wear. Compression wear do make an enormous difference believe me, they wick away the heat and moisture and you are far more comfortable for it! While layers can look pretty cool, especially if you are in a hot climate try and build your costume to
look like layers rather than just building them up. My normal Jedi has something like five layers around my torso and while all are cotton (except for my camisole I wear underneath) I get far far hotter in that than I do my Spartan foam armour which is compression wear under armour which has breathing spaces at all the joints! My new Jedi has combined three layers into one almost and will be much cooler and more comfortable, but when it is all together you can’t tell

You can also mount fans in your costume (my armour has them but I only need to turn them on when really hot) or you can get vests (or make your own) that allow you to fit in cooling packs. Fully covered costumers like furred creatures really benefit from these! Also make sure you listen to your body- take your helmet/ mask off regularly, go outside and get fresh air and if you are starting to feel light headed at all immediately take a break and shed some of the outfit if you can to cool off. As others have said make sure you hydrate regularly! If you be smart you can easily go all day in even a hot outfit- I’ve done 13 hours almost straight in my Spartan (con then the afterparty) and the only break out of the armour was the half hour from de-armouring, taking a short break, driving then armouring up at the afterparty.
Similarly dress for your climate/ the weather. As Justicar said, have an umbrella or poncho if it looks like rain- large well made capes can work brilliantly as well, my mum made everyone heavy lined capes for a medieval faire and while the capes got soaked from the constant drizzle and rain, we were almost completely dry underneath (cept the tops of our shoulders and bottom of our pant legs). If it is hot, consider a more breathable costume over a heavy insulating one. If you really want to wear that skimpy outfit and it is freezing maybe wear a shear bodysuit underneath? Dancers often wear them, they can come in different types and skin shades/ colours and unless you are REAL close you can't tell it is a body suit and not skin. Maybe incorporate a jacket or something in, even if it isn't as to character if you do a good job it'll look the part and be practical. Take spare 'civi' clothes to change into thatll either keep you warm or keep you cool depending on the weather!
Make sure you can go to the bathroom!! This might sound like a no brainer but its not something most consider. Make sure that your costume allows you to go to the bathroom, or know what is involved in removing parts to do so. For me in my armour I can get away with removing a couple of pieces- my husband has to take everything off. Remember one piece suits make this even more difficult! You might need to make sure you have someone to help you
Know your costume: Leading on from above know your costumes limits- test your mobility beforehand AND things like getting through a doorway! Can you
actually manage to sit down in it? If you can’t it will be a very very long day for you! Practice changing in an out of your costume beforehand, know what is involved and how long it will take. Walk around home in it, wear it while watching a movie! Get used to it and discover problems with it first so you can fix them. Smaller cons and hobby fairs are often the best place to trial run a costume as you are better off discovering the problem there than at a big ComicCon! When we first started costuming with the Halo armour it took more than half an hour to get into it, sort out problems etc- now we have cut that down to 15 minutes to get two people fully into armour from just body suits. Practice makes perfect!
Check the Cons Rules! Again a no brainer but don’t just wave off the policies with a huff only to get there and be denied entry. If they have weapons policies or codes of conduct READ THEM BEFORE GOING. If they say to tag your weapon, do so. If your weapon can fire darts or similar, remove any darts as well as firing mechanisms- so long as you do this and can prove it can’t fire they will (depending on the con) let you have nerf guns etc. Have foam or otherwise ‘soft’ props where you can, you don’t need to take a metal samurai sword in with you

. I will say, some con security are just a-holes. We have had costumers who are there as exhibitors told to go check in their weapons, only to have the check in people roll their eyes and say no you can go in with that because it is well within regulations and we say so. Regardless, be polite! Also remember the golden rule: COSPLAY IS
NOT CONSENT. Respect others and their personal space, ask before you take pictures. Don’t hug unless they expressly say you can and don’t do anything of a derogatory nature. Keep your hands to yourself and don’t say rude things, nor judge a person because you don’t like their costume or how it is made- even if that is ‘in character’. You wouldn’t want someone to do any of this to you especially if you have put a lot of time and effort into your costume. If everyone remembers to follow these rules then it makes for a much more pleasant experience for all.
Travel: As previously discussed consider your travel arrangements. Some people may find some costumes offensive or worrying. We always drive, and we usually get dressed in the carpark at cons (it is a very valid tactic

) and so we wear our undersuits/ under garments and then put the rest over the top when we get there. If you are using public transport or entering a place of business remember to remove hoods, helmets or masks- it is a common courtesy and in a lot of places a requirement (at least here, motorcyclists are told to remove helmets in any kind of shop).
Weapons and Props: Firstly keep an eye on them. If they sit in a holster or hang off your belt etc make sure they are secure because some people are
not nice and will try to steal them. This will also stop you accidentally losing them. Our Halo sidearms just sit on our thigh pieces with rare earth magnets which means they can be bumped off easily, mine is on a retractable string keyring which clips to my belt so if it does fall off it doesn't go far and can often click back into place on it's own. People also can't steal it (yes, they have tried) and I can still pose for photos with it attached just fine. Also be aware of your props, especially if big- don't go letting them fall, swing around behind you, etc as you could hit someone with them! Practice carrying them in a manner that keeps them close to you and out of other peoples way, I have found if walking with a single lightsaber a reverse grip keeping it close to my body often works well. Incidentally props CAN be handy if you get separated from your friends- I just hold my double bladed lightsaber high with it lit and hubby can usually find me from a fair way away!
Have set meeting points: When you arrive at a con with friends have a set meeting point that you will return to if you get separated, either by accident or because you have decided to go your own way for a bit. When in costume you can't always get to your phone, or even hear/ feel it ringing. Having a set meeting point is handy, as is having set times you will agree to meet back eg for lunch etc. It is also a good idea to have a rough idea of your schedule beforehand if you are going for photos or signatures at all and have you all know it. Designate someone as the time keeper! All of a sudden you might find three hours have passed really quickly!
Finally I’d just like to post this which popped up on one of my cosplay clubs facebook pages the other day:

Though I would REALLY like to know where they have gotten good costumes for $50!

I wish it was so cheap!