-Practice heavily after a round of a three day tournament at the course that provides free range balls, your skin will tear if you hit too many.
-Eat a hot Santiago's breakfast burrito (habanero pepper green chile) 20 minutes before your tee time
-Carry more than 14 clubs
-Take a drop at the point of where a shot went out of bounds
-Get tense or frustrated if you do worse than a practice round, it's normal.
-Assume that there is a DQ penalty attached to improper procedure while dropping the ball
-Count your clubs before a round
-Know the rules, and call an official if there is any doubt
-Carry a wet towel with you (A bath towel hung on the clubs, with half on each side, makes a great wet/dry towel combo if you only wet the side away from your legs)
-Mark your ball anywhere outside of 6"
-Carry plenty of tees, ball markers, and ball mark repair tools (the latter two are there in case someone forgets theirs, because I hate seeing balls marked with a tee or ball marks left on the green)
-Remember to relax and have fun playing the sport you enjoy.
Learned my lesson on the OB rule the hard way, because I played with some real jerk who convinced me it was a DQ penalty for not taking a proper OB drop (I was newer to golf then) so I dropped out of the 2nd day of the tournament. Didn't even bother coming back the next day to play the second round (they'd pair me with the first group, and just not count my score) since I knew I'd be paired with the same ignorant guy again. Never trust a guy who tries to give you advice, advice that matches what you already planned to do, says, "See, trust your pro" after it works, then complains when you give him the proper penalty for advice and proceeds to shoot a 107 with a handicap of 10. He even tried to hit it out of the ESA that was obviously full of snake dens after we had seen a rattlesnake on the course earlier...
However, don't let stuff like that get to you, and just have fun. That's the most important part of tournament play.