Seems like there is an abundance of advice here but, I may as well add my 2 cents. Noone else has mentioned this and i feel it has helped me tremendously. When I practice i put an enormous amount of pressure on me, after all i am playing the future greatest player on the planet, me. Thinking that i am playing 'greatest player on the planet', puts alot of pressure on myself to win. I don't actually believe that I am or ever will be the best player, but i try and get my mind in that mental state. Every shot means the world to me, in practice. I feel i should break and run every table, and I put 100% effort into doing that, even when playing by myself. When i go to a tournament i try and get into a mindset that tells me that every other player there is inferior. If you were to talk to me, you wouldnt see that, i still let players think that i fear them and show 100% respect, but in my head i try to be the most confident cocky person i can. Every time the other player misses a shot, or misses shape, i laugh inside my head and think how unworthy he is to be playing with me. When i miss a shot or shape myself, i forget about it instantly. By the time i get back to my chair I've forgotten why im sitting down again and laugh at his next miss (internally laugh). So my final words of wisdom are, PUT WAY MORE PRESSURE ON YOURSELF TO PERFORM WHEN YOU PRACTICE, AND CONVINCE YOURSELF THAT THE MORON AT THE TABLE DOESNT DESERVE TO BE IN THE SAME ROOM WITH YOU. Don't change anything on the outside though.
PS: I forgot one last thing, stay silent through your whole match. If someone is gonna talk to you, move. Go sit by yourself and don't open your mouth. The last thing you need to do is worry about the conversation you are having. If someone has advice to help you, tell them to save it for after your match.