Insoles and band aids are a life saver. Your feet with thank you.
A change of socks. [I do this when I go hiking as well. If you're walking around like crazy, you would be amazed how comfortable a new pair feels half ways through the day.]
Deodorant is a must, people will be able to tell if you're not wearing it, since sweat is inevitable. Also-- some cons even have rules against allowing people in that are unhygienic.
A makeup bag will save you, also consider bringing a needle and thread or a hotglue gun in case you run into some costume malfunctions. Some cons have booths that are dedicated to helping you repair your cosplay, but don't count on them being there.
Eat breakfast before going to the con. It'll curb your hunger and you won't be so tempted to being roped into overpriced food. My boyfriend and I usually bring a cooler and leave it in the car as well since the price for even a bottle of water is ridiculous at these gatherings. So we usually picnic out there when we get hungry.
The last thing I could tell you is don't spend all your money the first day. There's a lot of things you're going to want, but throughout the con date chances are you're going to kick yourself later when wandering the vendor area only to realize you already blew all your money the day before.