My number one piece of advise is to set realistic expectations and speak up when you don't think others are setting realistic expectations of you and those around you. This is especially true in the competitive and isolated environments of startups. I have been in many prolonged 100+ hour a week, deadline motivated design sessions that left me in severe burnout.
Everyone has different capacities for this but every human mind has a limit. It starts screaming for things it is not getting sitting in front of a computer monitor for 16 hours a day. My ability to abuse myself like this has dropped sharply as well as I have aged.