Do it today! This is something that is so obvious, we rarely ever do it. Make a list, post it on your blog. It's so life-affirming, that it can't help but make you happy.
Ignore the problems (read: opportunities) in your life, and think about all the good things, little and small.
Got your health? Be thankful! Have loved ones? You are truly blessed. Not starving or dying of horrible diseases? Praise life!
Your list can be as short or as long as you want, but when you're done, look over it, and give a little prayer of thanks (no matter whether you believe in a higher power or not). We are very lucky people -- and we should never forget it!