Here's a brief summary of a study on the economics of happiness. They isolated the following factors as those which had the largest positive correlation with happiness:

Highly Educated, Married, Female, Retired, High Income, Looking after the home, Young or Old (not middle-aged), Self-employed.

I was rather amused by how contradictory this list is. I guess the optimum situation is to be a highly educated, young, married, female, who is self-employed and making a lot of money working out of the home. Still that doesn't work in retired... I wonder how many people fit that profile? For example highly educated and young is not well correlated with married.