The Con suit is not a substitute for food! I know it's there and it's free and that you sometimes don't feel hungry- but you need food! Make sure to get Subway or even McDonalds, but you need to eat! Water is another big one, I remember seeing a girl put into an ambulance during the rave because she passed out from dehydration. Don't let that happen to you! Drink plenty of water!
As for uncomfortable costumes, take a break, sit down- especially if you're wearing heels. I got a little sick when I wore my 3 inch heels for the whole day, it really wasn't fun at all. So don't be afraid to sit down for a half hour if you're feeling uncomfortable or hot, or whatever.
Next Convention: Anime Detour
Blackfire- Teen Titans
Weiss Schnee (White Outfit)- RWBY