I'm going to start doing this. Cool little exercise that requires minimal time. I first heard this last year from LinkedIn's CEO and came across it again today on a cool forum.
Every day (at the very start or end) think of, and write down, 3 things that you're thankful for (it can be anything, no matter how trivial or minor. Keep an open log and track it over time.
Simple and easy. If you try it out and find value, let me know what you think! I'll be sharing my experience with this in the next couple weeks.