One thing I wish I did was being consistent with exercise at the beginning instead of finding excuses (read: drinking instead). JK.
Seriously though, it only takes like half an hour and you'll feel a lot better. You will look sexier in OPP lab. You also won't be a huge hypocrite when you tell all your patients to engage in 30 mins of physical activity 3-5 days a week.
Oh and another secret from a wizened upperclassman--study in the undergraduate library. Never, ever study in the library for your medical school. For me, that was my secret study spot for the first two years. It doesn't have to be the undergraduate library, just anywhere that's not the medical school's library. Things get too hectic around exam time with people freaking out, pulling all nighters and screwing around on FB. Studying away from that will help you to keep your mind right.