I am not so good at this. I have done at best 2 of the 5 things below:
1. Play games with the time control used in the tournament, especially if it is much faster or slower than your usual play. There are stories of players showing up at the KPMC not prepared for the 30+5x:30 time control because the tournaments in their home countries have longer limits.
2. Try to make sure that home and work crises are managed as best as possible beforehand. It's hard to play well when distracted. This is also easier said than done, but I feel it is still important. Even if it's just making sure the laundry is done.
3. What dfan said.

4. Try to find a cue to get into the zone. What have I done right before good games? Walk? Listen to specific music? Play a bunch of blitz? There is a lot of personal variation, but the idea is to get some repeatable ritual that works and doesn't take too long. There is a bit in Josh Waitzkin's 'The Art of Learning' on strategies he used and helped others develop.
5. Figure out how caffeinated you are going to be. Deciding to quit two days before is probably not wise.
