Honestly, I feel like a hypocrite talking about how not to burn out in med school when it happened to me numerous times. I guess experience is the best teacher.
I'll keep it short. Work out, read something that isn't medicine, hang out with friends and MEDITATE (!!!!!). All this, of course, is easier said than done. What I always felt was essential is figuring out what your study method is and making your study more efficient (again easier said than done-- do your research on this one). This will free up some blessed hours and enable you to do something fun.
What people always talk about is how one needs the willpower to keep on studying, which is true. However, no one ever talks about how much willpower it takes to stop studying, chill out and do something else. So it goes both ways, force yourself to study and force some down time as well.
P.S. The meditation part is big, helps manage stress. Remember that prolonged stress = prolonged exposure to cortisol = less memory retention + more irritability.
Know that many people went and go through it
Hope this helps.
cheers