Beer.
Beer is proof that God loves us and wants us to be happy.
That $350 mil winning lottery ticket. I’d buy a tropical island and party till the end.
Happiness is blowing away all of the people that make you unhappy !
Here endeth the lesson.
How very cinical:( .
I think the key is to lie to yourself about being happy until you grow to believe what you’ve been telling yourself is true.
I guess my key to happiness is a bit cinical too;) .
According to a study of Texas working women by Kahneman et al (not yet published?), some keys to happiness are:
-spending time with friends
-not spending too much time commuting
-appreciating the pleasant experiences that occur regularly
Also according to Kahneman (Kahneman et al 1993): The key to having happy memories of an experience is having a very good 1) ending and 2) high point. The rest of the experience doesn’t matter. The duration doesn’t matter. It’s called the peak-end rule. [/sexual advice]
Most importantly:
Low expectations. Don’t worry about always making the best possible decision. Be satisfied with a good enough decision. “Satisficers” have been experimentally shown to be happier than “maximizers,” and much less likely to be clinically depressed. (sorry, no cite)
What is not the key to happiness:
$$$- Once you’re out of poverty, more money doesn’t make much of a difference. Lottery winners are really happy at first, but within a few months after winning the lottery they are no happier than the average person (Brickman, Coates, & Janoff-Bulman, 1978). Also, paraplegics are nearly as happy as everyone else within a few months of becoming paraplegic. People adjust to their circumstances much more than they expect to. This adjustment has been called the “hedonic treadmill”.
Forgive
Give
Love without expectation of return
Read a lot and take lots of long hot baths whilst reading good books. That is really how I manage to do the previous three answers. When I manage it, that is.
For me personally, and in order of preference, peace and goodwill between us all; failing that, money, the ‘lots and lots’ kind.
Hmmmm. No wonder I’m so depressed…
I second this. Once I learned to let go, I have been happy ever since.
Your key to happiness may be different to mine, but the above is what worked for me. 
There’s already a lot of good advice here, so I’ll just add that ONE key to happiness is recognizing the difference between pleasure and happiness.
lobley,
I don’t think that Lord Buddha said that you must give up all your possessions to attain happiness. The idea is that you must not be attached to the possessions. You can be attached to something you do not own. The second Noble Truth says that the cause of suffering is tanha or desire. Give up desire and you will be awake. And happy.
I’ve always told myself I only needed three things to be happy:
- To find and keep a job that I love doing,
- To fall in love with and spend the rest of my life with someone who feels the same way about me,
and - To get treatment for my depression.
I don’t have any of them yet… working on #3, but it isn’t going too well. 
It does my little psychologist heart good to see someone reading the literature. 
The bottom line on the studies with lottery winners and paraplegics is that it is not what happens to you, but how you think about it. I work with people who have newly aquired spinal cord injuries. Research has shown (and my experience has confirmed this to me) that most of them are not depressed. Furthermore, the ones who are depressed are not the ones with the most serious injuries. What differentiates between them is what they think: “This is a tragedy. My life is over” vs. “I wish this hadn’t happened, but I will survive.”
What is best in life, you ask? To crush your enemies, see them driven before you, and to hear the lamentation of their women!
Self-acceptance.
That gives you peace of mind.
Of course, real self-acceptance can only come from being true to yourself (which I also find straightforward, logical and useful, rather than a cliche).