Sports usually seem to do the trick for me a lot. Also, going to the gym is a good way. The hardest part is just getting to being physically fit, it's easy to maintain once you're there though. Also, it depends on where you are physically right now. If you want to lose weight, running is a pretty good way to do that. If you want to gain muscle, just doing some work out plans can accomplish that. Or if you want to be strong and have a really nice body, working out and lifting weights accomplishes that.