Recall the good things that happened to you in the past. If you mentally relive those experiences, you will be in a much better state of mind to deal with the present more efficiently.
Keep a list of the good things that have happened to you. When you feel sad, take out your list and read it over. Think especially about those things you felt joy over when you first obtained them, things you still have. By recalling your original joy, you will feel better now.
Sad people tend to talk about their misfortunes and this causes them needless misery. They should form the habit of talking about the positive experiences of their life. When you talk about positive experiences and thoughts, they have a positive effect on your emotional state. By doing this a few times, you build up your confidence in your ability to evoke positive emotions.
(Hishtapchus Hanefesh; Rabbi Pliskin's Gateway to Happiness, p.181)