Most people have covered the main stuff: comfort, seeing game, developing mental toughness.
A couple of practical things I do to sit all day are:
1. Those little energy drinks. I know some won't agree, but they keep me alert.
2. Give yourself something to look forward to. Tell yourself you get to eat your snacks at 8:00, 10:00, noon, 2:00, etc.
3. Think about the time when.... I've had it happen a number of times that the best deer movement came after seeing nothing for hours and I could barely sit anymore.
4. Do some calling. For whatever reason, giving a little rattle or grunt (at the right time of year of course) gives me the anticipation I need to sit it out another half-hour.