The basic everyone else mentioned (sleep, diet) + minimizing negative influences: especially alcohol (which mess heavily with both sleep and digestion), negativ

The basic everyone else mentioned (sleep, diet) + minimizing negative influences: especially alcohol (which mess heavily with both sleep and digestion), negative thought patterns and ... negative people. Sleep conditions matter also -- it needs to be quiet, restful sleep, with the windows open if at all possible. Cutting down on screen time (i.e. this thing you're looking at right now), in favor of book time, or just-about-anything-time if its happening, you know, outdoors and in beautiful scenery, helps also.