This question was addressed in a large-scale project by Gretchen Rubin who tried to increase her happiness over the course of a year by setting herself specific goals that covered, among other areas, marriage, work, leisure, parenthood, friendship, money and so on.

FORBES: Happiness Is Hard Work

If anything the U.S. Task Force has done has given this child one year of peace and happiness, then my stay here has been worth it and I can count my blessings this Thanksgiving Day.

CNN: What are you thankful for?

One truly compelling piece of data he offers: lottery winners and parapelegics, one year after the event (either winning the lottery or losing the use of their legs) report nearly identical levels of happiness.

FORBES: How To Be Happy, No Matter What