Have the courage to stop, sleep, eat and then come back with a fresh mind.
I find it incredibly sad that I even need to think about this. I regret that I even participated in this circus in the first place. I now only work for companies with don't have the ass-in-seats-for-80h policy. It's actually interesting that in startups where this works, people self-select and end up there. It's amazing how much more you can achieve if you can afford to take a step back, and not worry how you can run in circles even farther and faster.
There is a lot of value in having someone who is always available. That is not me since I work in spurts, with long breaks. I think lack of always on, immediate availability is important for some projects, and I am unwilling to do that.