The most useful tool I've only started to learn is to focus on PRODUCTIVE hours of work per day, rather than total hours spent working. I assume everyone's different, but for me six hours of truly productive coding in one day counts as a big success (yes, I CAN get more in, but for how many days in a row?)

Realizing and accepting when you're no longer productive and switching tracks is one of the most useful skills.

Startup founders have a lot of things working against them - the flexibility to set your own work schedule can be one of the most powerful tools. If this involves taking a mid-day break to run around outside for an hour, or going out on a weeknight and sleeping in the next morning, or working throughout a weekend because it makes sense, then go for it. With the right balance of discipline, it'll keep you happier, it'll keep you sane, and it'll keep you more productive - per hour spent working - than anyone in a cubicle.