When I play I focus on each microdecision. Raise/bet/call/fold on every street... what is best? what do I know from previous action? what ranges can these opponents have? Always focused.
All problems are in our heads. When I start to play, I try to remove from the field of vision and audibility everything that can somehow interfere with me. And I completely immerse myself in the game.
All I'm focused on when I'm playing is that I should keep myself close to the average stack at every tournament, if I fall half of the average, I sometimes go for an all in pre-flop.
It may sound simple or repetitive, but it would be to eat well and exercise, the question is to be able to keep clear for those long sessions, I play my favorite music and that helps me stay focused.