Personally, if it's the pressure that gets me, I remind myself that it's just a game and that failure is not the end of the world, and try to focus on playing well rather than winning (if that makes sense).
I.e. when I'm scared to loose I tense up and start to hesitate, but if I focus on my footwork, balance, stroke and tactics I start to relax more. If I loose a ball I try to pep-talk myself and remind myself what I did well, like "I might have missed that shot but it was the right call to go for it".
And like the some other posts in this thread already mentioned, the data from the previous sets are crucial. What has been working for you and what hasn't? How is your opponent winning points?
Usually, I try to save some tactic that I haven't used in previous sets, e.g. some weird serve or a new serve-attack combo. Your opponent are most likely going to be a bit tense as well, and bringing something new into the game is hopefully going to make them more stressed
