Try the below suggestions or at least consider them if you get nervous before or during a match - that's what I am going to do.
Breathing - take deep breaths to slow the heart rate down. A little adrenaline is okay (and normal), but too much adrenaline can be slowed down with deep breaths. Breathe in and hold your breath. Then release very slowly. Deep breathing has been well documented to ease stress and ease anxiety.
Challenge - view any tough opponent as an opportunity! As a challenge! Welcome the chance to play them and test your skills. Reverse any fears of a tough opponent by viewing it as another great experience. Just *thinking* that way can already slow the heart rate down because you change your negative/fear thought into a positive thought.
Act - Standing tall, sitting up straight, and walking around the table acting confident even when you are not... is amazing how it transforms into real confidence! Almost exaggerate these "actions" if you have to Acting like you have confidence truly does transform into confidence. Along the same lines of smiling to make yourself feel happy, acting confidence breeds confidence (try it, you will be surprised!)
Focus on Your Pre Shot Routine - this is the biggest advice. If you are focusing completely on your fundamentals, and your pre shot routine, your mind has no room for thinking or roaming. Phil Capelle taught me this when I struggled with embarrassment a few years ago. It's very, very important to do this. Mika also recently wrote a whole blog about what to do under pressure - breathe and focus on your fundamentals!
Have Fun - enjoy the moment, the table, the decisions, the environment. Enjoy playing the game you love.
Early Recognition - This is crucial - recognize early if negative emotions are affecting your match. As soon as you recognize what is going on (not staying down, thinking of the "what ifs," getting frustrated, etc.) the sooner you can reverse the thinking and start playing pool again.
Perspective - if you are nervous, try to put things into perspective. Get your mind thinking of the obvious - it's just a GAME. Yes - it means A LOT to you - but fight the nervousness by putting the situation in perspective: this isn't life or death, you aren't in a car accident, you aren't going die of embarrassment if you fall on your face. Everyone has bad days - you are human, it's okay.
Awe - fill yourself with awe. Be thankful you can even play pool, can attend the tournament, have the opportunity to be in this event. Nothing fills the heart more than realizing, "hey, I'm so blessed to be able to play pool, my passion in life."
Trust Your Stick - give it your all, but trust your stick too. You practiced hard now go out there, have fun, and play your game!
Reverse It - if your opponent upsets you, turn it into a spur for your game - make it become a motivating factor.
Jumping Jacks - got into the bathroom during your break and get that blood flowing! (don't slip, though.)
Stay in the Present - don't psyche yourself out by already playing two future matches in your head while you are playing a weak player. Don't think of future results - the game isn't over yet! Don't psyche yourself out by thinking everyone is thinking you're dogging it. Don't psyche yourself out by relaxing your fight if you are ahead in the match. Bear own, but bear down on the shot right in front of you.
Play Your Best - give your best effort for every shot. Don't rush easy shots or take for granted a wide open table. Play your best every single shot. Make the most when you are at the table. Don't let up at any point. Focus on playing your best to contain the nerves.
Focus on What You Can Control - focusing on things you can't control takes your mind away from the present. Did they crap in a ball? Is the A/C broken? Well, you can't control these things - why let them bother you? Worried about what others are thinking? They are probably fighting their own mental demons and not even thinking about you, lol. Your mental approach and your pool game are things you CAN control.
It's a Good Thing! - nervousness means you care - that's a good thing! Remember also, that it's natural to be nervous; natural for the adrenaline to flow. Don't fight it - accept it and feel it for what it means - it means you love the game. Embrace it.
Remember - winning or losing doesn't define you as a player or person.