Hydrate Hydrate Hydrate.
Ever since I joined the military it is something that has been drilled into my head.
As mentioned before The arid air at these cons can dehydrate easily. Also as mentioned before drink lots of WATER.
Now to add a few things from a military perspective.
Energy drinks are BAD. They are a diuretic which only works faster to dehydrate you.
Soda. see above. also bad for this very reason.
Gatorade is something you should drink to Rehydrate and replenish. meaning its for after strenuous activities workouts etc. walking around is not strenuous.
Your probably thinking well Im in a fursuit and getting water in here is a challenge. Non suiters this will work for you too.
Look for something called a CAMELBAK. If I can fit this under my helmet and on my flak jacket you can surely fit it inside your fursuit.
Most sporting goods stores should have them and are well worth the money.
see examples.
They cost anywhere from 15 to 50 dollars.
You do not have to buy the camelbak brand there are off brands that are relatively cheaper. I will say this. Off brands will and do tend to leak.
however if you do not want to buy a camel back keep that bottle from the soda you just drank handy. or a canteen.
just a note DO NOT exceed more than 12 quarts of water per day. You can get water poisoning / intoxication also known as hyponatremia. Not only is this not fun but it can be fatal.
Hope this helps.
-Arashi
What an awesome little (well...) item. Thanks for posting. We get several cases of heat prostration/dehydration every year; this should help.
One 8-ounce Gatorade should be enough to last you throughout the day. Drink plenty of water thereafter. The Westin Hotel provides NUMEROUS pitchers of water for everyone. If you have even the slightest hint of dry mouth, then go grab a glass of water. If you start feeling dizzy or nauseous, then it's time to get out of costume or head straight to a headless lounge.
Some of you think it's funny, but I highly do NOT recommend drinking alcohol when in costume. This is extremely dangerous to your health and your well being. Alcohol will deplete water from your system far faster than energy drinks even will.
I know more than most people what the meaning of staying hydrated is. 
I personally recommend between 8-10 glasses of water per day for every single person, 10 or more for people in costume.
Powerade tends to help a but because it not only keeps you Hydrated but it contains a Decent ammount of Vitamin B in it to keep you energy up.. But If you are feeling cheap, add a pinch of salt to your camelpack's water supply to keep you Hydrated.
That is definitely some good information for those that are unaware of the various effects of what you drink. Me, I just plan on having my spotter follow me around with a firehose.