Just do it. And no need to wait for it to come to you naturally.
Bad day? Call a witty friend. Call one of those people in your life that you just laugh until you cry with. Search Youtube for "baby laughing", "dog singing" or anything that tickles your fancy! Pop in your favorite movie, google a joke of the day site, actually read one of your friends forwards or in all necessary measures...go to a comedy club.
Laughter really is the best medicine.