If you hold 15 seconds of positive thought, just four times per day, this will give you massive power.
Do this right now, think of a time when you were happy. Purely in the present moment and enjoying life. Don't over think this. It can be something simple, like having a drink with a good friend at the pub. Go back to that point, and experience it again. In slow motion. See it all happening again. Just Allow yourself to feel good. Really good.
Do this just four times per day. Then if you feel like it, if you want to, go ahead and hold it for longer. I've found the longest you can stay with a thought and feel really good is like 3-4 minutes - so then just choose another. Allow yourself to feel really good for as long as you'd like. But don't force it, just do it if it feels right.
Interesting. I just started doing this yesterday. I can confirm the power.