I'm a Darts player. And in the sport of darts 80% of the game is mental. When in a match your adrenaline goes up, it heightens your concentration. After the match the adrenaline drops fast, like coming down from a sugar rush. Your concentration and focus is off. making it hard to get your self ready for your next game.
What this has to do with Poker? Well, because the same method's I use in Darts to keep my concentration and focus. I apply to poker.
1) I never stop playing every hand on the table (even when I'm not in them) this helps me keep a strong read on all the players on the table. My mind doesn't wonder.
2) I never listen to music, text or any thing that will distract me from the game. Nothing more then listening to music, or other distraction can take me out of a game and loose my concentration and focus. (And put me to sleep)
3) If/when I do loose my mental sharpness (concentration) I take a break. Go to the bathroom, walk out side. Anything that gets me away from the table for a few minutes together my thoughts. Reminding yourself of why you are there and what your goal is, is the best way to recharge that battery and keep you mentally sharp.
This helps me when I play in very long sessions. Staying mentally sharp is just that, mental. Your mind always needs something to do. If it's not doing something it's sleeping. So if you train your mind to stay focused. You will stay mentally sharp.