Knowing it's going to be a rollercoaster really helps -- you don't get too caught up in the highs, and you know the lows are only temporary.
Sure you're still going to have them - but being conscious and aware makes it a lot easier.
Don't overwork yourself. Get at least 7 hours of sleep. Exercise at least 3 times a week. Keep a pulse of your venture and know when to move on if things aren't looking good. Life is short so don't kill yourself with one thing, you must iterate. Make time for family and friends.
These are some of the things I've learned over the years. Please add to this list if you think of anything else.