1. If you get nerves, don't drink coffee within a couple hours of the tournament.
2. Lots of water - hydrate, so your muscles don't start doing weird things when the pressure is on.
3. Don't drink to calm your nerves. It will work, but it will kill your reaction speed.
4. Headphones helps for a lot of people. You can even just wear the headphones with no music and it will shut up the people asking questions in the middle of your game because they'll assume you can't hear them.
5. Focus just on your next shot or objective. If the other guy has 50M and you only have 5M, the only way you catch up is one shot at a time.
6. If a shot isn't working, don't force it. On Monster Bash, for example, sometimes right Bride ramp is just a non-starter, or right wolf man orbit. If creature isn't working either, I shoot the spinner all day. Maybe once you've got decent points on the board, try that hard shot again.
7. Don't be afraid to chat with other players about the game, but it's not always considered cool to do this with your match opponent, and never cool to talk to them while playing. While I'm waiting for my turn on Game A and not really watching closely because I know it well already, I will watch other games. I'll watch how the game is feeding from kickouts or half ramp shots, or how tight the tilt appears to be, or if the mystery awards are on tournament mode, etc. If a player is playing a different strat than my own, I'll ask him/her about it. "You didn't go for x...." "Yeah, I prefer to go for y because..."
8. A mental exercise you can try: at your next tournament, when your nerves are off the charts, take a "mental snapshot". Look around, remember the visuals, the games, the score display, the lights, the other players, the sounds, the smell of the bar, the feel of the floor beneath your feet. Really burn it into your brain. Next, when you're playing casually, close your eyes and vividly recall that memory, and then play a game and imagine you're playing against someone right at that moment. Sometimes it even helps to start a two-player game and play against yourself. If done correctly it can help get you used to playing "in fear" and maintain focus despite the pressure.
Hope this helps.