Happiness is a stupid feeling which you can achieve only by ignorance and living in the moment only.
I m the happiest when I help someone really needy and make him feel happy. BTW I`ve only got 4-5 chances to do so in my 25 years life. And believe me nothing beats that feeling. You really feel good bout yourself and your life. You forget how many bad things you`ve done and how much life sucks. No one`s perfect. We all make some mistakes. Some bad, some very bad and some downright ugly.
May be some of you might not have done that BAD but I`ve personally done some extremely stupid things which make me feel bad enough to wish not being alive at all. But fact remains that I m not dead yet and I`ll definitely die someday, no matter what. And being an atheist helps me realize the importance of life. So there is no point in wasting your life feeling sad or bad bout anything. Be happy instead. And to achieve happiness you`ll have to be ignorant to those things/memories which you cant change.
Once you leave behind those sad memories, you can find happiness in even the smallest of things. It can be anything from having your favorite food, to watching your team winning against rivals, to promotion, to knowing that 10" MBP is launched(even if you have no intentions/money to buy one. ever). And yes, not to forget, back to back sex.
EDIT: WOW. I just remembered I had a dream last night bout finding a 17" MBP in my room with no takers. That thing felt huge compared to my MB. And I cant tell how much happy I was. I even remember checking out the KeyB light. It looked so cool.