I focus on my breathing, on the ball, & on enjoying playing. I couldn't care less whether I win or lose as long as I have played my best. The less I care, the better I am able to play.
I periodically (before serving, after a point) briefly close my eyes and smile to stay realxed. I do not interact at all with the other player - I play in silence - drives some opponents nuts but I feel relaxed and within myself and more able to regulate my level of intensity.
I also have a set routine for serving (breathing, 4 bounces of the ball) that I never vary. Also - whenever possible, I make sure I have enough time before the match to do a good dynamic warmup and then relax by myself for a little while before playing.
Life being what it is (I hate my job, hate my job....) some days I am too wired and then it is hard to be calm enough. Otherwise, it really does help.