redlordofvampires makes a very crucial first point, this isn't your local gaming store, first things first, keep your things with you! Don't set them down and turn your back. If you have a binder or something you don't want on the table or can't have on the table a good idea is to make sure it has a strap of some kind that you can wrap around your foot or ankle, that way, you won't accidentally get up and walk away without it.
As he also mentioned, a lot of the cheating that is done happens when you're not paying attention to your opponent. When you're writing down a score, examining your hand, searching your deck, etc. Keep one eye on your opponent and the other on what you're doing, or glance up and down often.
Compare scores with your opponent on occassion to insure all is well there.
Announce your phases, even if it sometimes seems "newbish" This can keep you from a judgment call when it comes to a he said / she said scenario.
Dance with the lady what brought you. (Old timers saying) What that translates into is, "Don't decide to play a new deck the day before or the week before the SJC" Players have a habit of second guessing themself, don't. Your best bet is a deck you're very familiar with. It's alright to tweak your side deck for what you may anticipate so you have a counter but be careful about comprimising your deck's normal strategy just for this tournament.
Get some rest! Contrary to what anyone may tell you, a well rested player is a more alert and metally prepared player.
Along those same lines, be prepared in all ways. Have 3 copies of a typed deck sheet ready. One for when they're collected, another for your deck box (I'm constantly amazed at the number of players who don't keep a copy of their own deck sheet and ask judges to look at their deck sheet because they can't remember what is in thier side deck and therefore can't de-side deck correctly without it) Have some money, 60 new sleeves, make sure you current sleeves are identical and in good order, dice, a couple of coins, fusion deck, side deck, replacements for as many of your cards as you can (in case you lose a copy for some reason) 3 score pads, a couple of pencils, calculator, tokens, etc.
Keep track of both your score and your opponent's score ON PAPER. A judge will almost always go with a written score in instances of a life point discrepancy.
Don't get discouraged, the first time is always a learning experience. USE it as such, don't drop, play the whole thing through. You paid for a whole tournament, enjoy it all.
Pay attention to announcements. You'll likely not have a lot of time between rounds. Don't wander off and miss the start of the next round.
Before you leave the table after a match make sure you de-side deck. It's easy to get excited about a great match your just won and forget to do so. By making a habit of doing so BEFORE you leave the table your deck will be ready for the next match. Along with that, before you leave the table count your deck to make sure your opponent didn't just walk off with one of your cards by accident (or on purpose) Take a minute to make a couple of notes to yourself at this time as well about what went right / wrong to review later. What did he / she do that gave you trouble, what did you side deck that saved you? What card(s) seemed to always be dead in your hand? These notes may very well come in handy later on when you tweak your deck or side deck.
Don't be afraid to talk with your opponent breifly afterwards. Most will, especially if they beat you. Obviously if they did they did something right. Tell them "Fine job, well done. May I ask a question? When did you know you had me? Did I tip anything off? Any advice you have would be much appreciated."
If there is a questionable situation call for a judge. Don't take the word of your opponent on a questionable scenario. Just tell your opponent, "You may very well be right, I hope you don't mind but I'm going to let a judge make the call on this."
If you feel the floor judge has made an incorrect call don't be afraid to appeal to the HJ. Do so of course with respect and dignity toward the floor judge. Listen to their ruling, if you don't understand it ask them to explain why they ruled as they did. If you still don't agree with it state, "Thank you for your ruling, however, I'm not sure I agree with it and I'd like to appeal to the HJ please." After the appeal, thank both judges for their time and effort.
When all is said and done, before you leave, make more notes. While we can all offer you advice none of it is going to be as important and is going to be as applicable as what advice YOU have for YOURSELF the next time!