I used to do at least 20 pushups and situps every night before bed, and then a run around the block every morning. I would try to go a little further every day. I'm too busy now to do it every day, but I try to do things like that as often as I can.
Also, getting a heavy stick and smashing the armadillo out of a tree or something is good for lean muscle mass in your arms and it is really cathartic.