I've never been to a Pokemon tournament, but hopefully the experiences I had going to a two-day state Quiz Bowl tournament apply here.
Definitely take lots of pictures in line with Rule Six. You never know if you're going to get back, and there's always a few people you'll hang with who you know it's going to be their last trip. Also rule two. Make sure you have cash in your pocket, you owe it to yourself to hit the town.
Always, ALWAYS fuck around in the hotel room when you're not busy kicking ass and taking names. Trying to win a trophy is good, but making memories you KNOW you're going to cherish for years to come is even better. Besides, it'll take the edge off of drawing that grass energy and proceeding to get haxed to death.
Oh god, rule three. Granted, my matches tended to take place at high schools and colleges instead of cramped convention centers, but this rule CANNOT be stressed enough. There will always be some people that don't adhere to this rule, but better to be part of the solution than part of the problem, right?