A good nights sleep is by far the most important. When you get there
a. Don't play blitz, but do a few puzzles carefully so you get used to looking at the whole board and concentrating.
b. Take deep breaths or something before your move to manually focus yourself.
c. Bring a lunch or at least some snacks. Depending on the time control, some games can be very long and having a bag of carrots or an apple joice is a useful for staving off hunger until the game is over.
d. When you see a good move, ask yourself, is there a better move? one perhaps that better fits into your strategy or that mates quicker. It's excellent to block a threat, but it's often better to block/parry a threat with an attack on the oppositions king or queen.