Another peice of advice is bring everything you think you might need. I haven't seen it mentioned yet, but I think its a big deal. So if you run a card involving tokens, bring tokens, if you need dice bring dice. Don't count on your opponent to bring what you might need to play.
Also, don't be a dirty player. I know a player who intentionally left his opponents
Vampire Lord in the graveyard for two turns to get a double gameloss applied to his opponent. I know a guy who intentionally has scratched sleeves when shuffling the deck to get a game loss on his opponent for marked cards.
Don't stall, but don't rush yourself either. A good slow pace is best, but you shouldn't have to read, and re-read your cards every turn.
Don't trade while you're entered in the regional. If you drop(which I don't recommend) or when its over, go ahead. However, your trade stuff is just too much to keep up with because, if you're like my team, we have two big binders.
Don't be intimidated. I know a player whose won 7 regional mats, and plays on top of all 7 of them just to intimidate thier opponent. IMO, If the player needs to do that, then they have self-confidence problems anyway. Also, IMO a regional mat means they played well on that day, and it has little or no baring on that day. Thats a good attitude to look at it with.
Don't expect to Top 8 your first time. Cases where you top 8 on the first time are rare, and everyone is shooting for that slot. First time, you're probably going to get "stage fright" or "star struck" depending on how big the tournament is. Unless you got ice running through your veins this is going to affect you first time. This comes back to keeping a cool head and relaxing.
Food ! ! ! I like the energy bars suggestion, but some people don't like those. Upon arrivial find out if foods there, and always be aware of time. Don't Don't DON'T
DON'T bring food to the table. Judges will give a you a game loss, (and if they don't I think they should), and don't put drinks on the table. Its too easy to spill, and even with card sleeves, one bad soda could ruin someone's deck or stuff.
Be nice to the little kids. Not everyone at the Tournament is as serious as everyone else. Sometimes the way people treat the children just makes me sick, show them a little respect. Don't try to rip them for trades, don't ignore them(unless you're in a match), and don't yell at them. Its just wrong.
I hope I helped you. These things will hopefully help make your tourney day better. Good luck.