Don't eat sugar. Don't smoke. Don't drink. Eat fruits and vegetables. Exercise regularly. Walking counts, but you probably want to throw some upper-body stuff in there as well. Working in the garden counts.
That's about 80-95% of "how to stay healthy using forces under your control" as far as I can tell.
“The surprising finding is that our relationships and how happy we are in our relationships has a powerful influence on our health,” said Robert Waldinger, director of the study, a psychiatrist at Massachusetts General Hospital and a professor of psychiatry at Harvard Medical School. “Taking care of your body is important, but tending to your relationships is a form of self-care too. That, I think, is the revelation.”
Oh but where's the fun in that? :)