Treat the poker tournament like a job. Live tournaments and online majors can be huge grinds. Were talking anywhere between 8 and 20 hours, depending on the structure of the tournament. Being able to stay focused for such a lengthy period of time will require you to be on point at all times. Keep your distractions to a minimum. Live, try to only take bathroom breaks and meals or snacks during the allotted break times given during the tournament.
Eating a meal at the table or running to take a leak may not seem like a huge deal, but that salad you picked up to munch on will require your attention for the 10-15 minutes it takes to scarf down. When at the table, you should be focused on poker and poker only; I dont even recommend music players at the tables anymore, unless its something you must have to keep a clear head with. Online, try to keep only poker related things up on your desktop; no facebook, no downloading movies, etc. The less outside sources you have clogging up your mind, the more focused you can be for a grueling day.
Keeping the flow. Youre probably used to high and low points during your runs playing poker; big suckouts, massive pots, and the like. If you allow the adrenaline of those moments to flood through you every single time they happen, youll grind down well before the end of the day, and quickly lose energy and focus that are vital to the late day endgame.