I think the biggest thing is using your breaks properly. Get away from the screen, walk around, drink some water and clear your head. After playing for a few hours, it is easy to become tired and start making decisions without thinking as clearly as you did at the beginning. I also try not to spend the whole break thinking about the hands I just played, especially if I lost a big pot or made a mistake. There is no point carrying that frustration into the next level. The break is a good opportunity to reset, relax for a few minutes and come back with a fresh mind. For me, a good break can make a big difference, especially in long tournaments. Staying focused until the end is not easy, so using those few minutes wisely can help you maintain your concentration and patience.
I think physical condition matters more than people realize in long tournaments. If I feel tired, my decisions become slower and I start missing small things. I try to stay active between tournaments and avoid eating too much before a long session. It makes it easier to stay sharp when the tournament gets deep.