Comfortable shoes made for walking on concrete are the best. Your regular day-to-day shoes may not cut it. Trust me... there is a difference.
If you are cosplaying, unless it is a ridiculously looking over-accentuated Anime style weapon, don't bring it. If you have doubts about it - ask here before you bring it to the Con.
Socks - you may consider changing your socks halfway through the day. Buy a pack of new ones and throw the old ones away if you don't want to carry around stinky used socks all day.
Deodorant and clean clothes - they're not optional.
Make sure all the naughty bits are covered. Please... ask a friend for help with this. You may look great in the mirror, but at a distance or from another angle, you may leave more exposed than intended or what convention staff considers decent. (I have seen a lot of people with shirts that are long enough when their arms are down but reach for something, or after a bit of walking, the bottom of their gut hangs out. If you don't mind this, then you are ok. But if it is not intended, then ask a friend if what you are wearing covers everything you intended.)