Exercise has been suggested numerous times, and I agree. I'd also add to choose a sport where you have to focus your mind (i.e. not go to the gym or jogging).

Exercise has been suggested numerous times, and I agree. I'd also add to choose a sport where you have to focus your mind (i.e. not go to the gym or jogging). I have been doing downhill/freeride mountainbiking and enjoy bouldering right now. Both sports don't allow your mind to wander off - you need to be 100% focused on your body. In case of mountainbiking, a lot of adrenaline was involved because of the risk. I felt that when the mind realizes there are serious consequences from not focusing completely on the task at hand (e.g. when you are racing through trees), it will focus automatically. This hasn't happened to me when going jogging or so - my mind would still be able to wander off. YMMV, as always. Good luck!