1. Be prepared. (See lots of other great bloggers' posts on this-- look on the far right column at my blog's link list, there's one titled "Best ever tournament prep collection.") Don't waste energy stressing over stuff. Sometimes it helps to have a little index card with some things you want to make happen... "Grip fight, seoi nage, double leg, straight back, staggered stance, heavy hips."
2. Bring music. Listen to it.
3. Work the tournament (scorer, assistant to director, photographer, whatever.) It helps me take my mind off my nerves and makes me feel more at home.
4. Lay hands on your opponents (I mean in a friendly way, like shake their hands!) so that they're human and real to you. Sometimes it helps to feel their cold fingers or sweaty palms, it reminds you they're nervous too. And sometimes it establishes, subconsciously, that you are comfortable and welcoming them to your turf instead of the other way around. I know it makes me a little nervous when I see girls with this mean face on before a match, but I'm even MORE skeeredy-cat when they're all relaxed and happy and cheerful. Like, what does she know that I don't? So I like to think I can put a little of that worriedness into someone else's heart ;)
5. Know your coach's voice and be listening for it intently. Surprisingly most of the time it won't get drowned out. Occasionally I get really adrenaline-y and then my coach is like the Far Side cartoon-- Blah blah blah Georgette, blah blah Georgette-- but mostly his voice just sings right into my brain. BUT-- also listen to your opponent's corner, as often they're telling YOU what your OPPONENT is about to do. Quite handy ;)
6. HAVE FUN. No one else cares whether you win or lose, for reals, until you are bigtime like Marcelo or Roger. No one hardly even notices. Comfort yourself with the ultimate minimalism of your match-- it's a very important 5 or 6 or 7 minutes and is only important for about that much longer after the end. Then it becomes just a laundry list of the things you need to work on. :)