Shower and wear deodorant. Please. All of us suffer because of the few who do not observe these basic hygiene principles.
Stay hydrated, and wear a costume that is weather-appropriate, ie you won't die in your costume from heatstroke.
Don't spend more in the dealer's hall than you can afford. Set a budget for yourself in advance.
Bring a DS or PSP for downtime and line-waiting.
Don't forget to keep all your electronic gadgets charged. I try to give all of mine a full charge when I sleep so I don't have to worry too much about running out of juice in the middle of the day.
Don't go barefoot. It's illegal and dangerous to yourself.
Clean up after yourself if you make a mess of any sort.
Be considerate of others, and don't annoy or bother others. You expect everyone else to allow you to have a good time, don't you? They expect the same courtesy. Basically, apply the golden rule to every situation
Don't block the halls or entrances. People need to get through them, especially the people who are running the convention. The halls and doorways are also fire lanes so blocking them is illegal.
Please be patient with the staff. They might sound like they're yelling at you but they are probably just trying to yell over a loud crowd of people. Don't take it personally. If they are rude to you, it might be because they are running on too little sleep. Our staff inevitably do that. But if you can't brush it off, make sure you get their badge name and report them to the Info Desk or another staffer.
Some of these are rules in the convention policies so you may be stopped if you aren't observing them. IE if you are harassing someone else or if you are barefoot, probably not if you don't have a DS though.