Went and found the New Scientist article on happiness which I'd read a while back - I'd mentioned it to someone and wanted to make sure I had my facts right! I did (basically).

What Science Says...

The New Scientist article "Reasons to be cheerful" ( used to have a free trial registration, but at the moment it's subscribers only) lists 10 factors which scientists have identified as being important to happiness.

1. Have enough money to live. Having good status is more important than cash - it's relative income that makes the main difference
2. Desire less. A good Stoic principle. To quote New Scientist: In surveys by the Roper polling organisation in 1978 and 1994, for example, Americans were asked to list the material goods they thought important to "the good life". The researchers found that the more of these goods people already had, the longer their list was, so the good life remained always just out of reach.
3. Intelligence (in terms of IQ) isn't a factor.
4. Your genes account for a large chunk of your general level of happiness. Also, extroverts are happier, and happiness makes people more extrovert
5. Think of yourself as attractive.
6. Have a good network of friends / family
7. Get married
8. Believe in a religion
9. Be altruistic
10. Be old

What I say

1. Do happy things, think happy thoughts. This is a simple principle, but it's the principle behind "Cognitive-Behavioural" approaches, which are proving to be the most effective in treating illnesses such as depression, social phobias and even psychosis (BMJ article). So, listen to some happy music!
2. Ensure you're eating enough Omega Three Fatty Acids. Basically, the membranes in your brain cells are constructed of fats. If your body has these "Omega-3" fats available it uses a high proportion of these to build the cell walls. If not (e.g. you're living on junk food) then it will use the other fats available (e.g. those from that Mc Donalds you ate...). There's a lot of evidence that adding Omega-3 to your diet makes you more intelligent, calmer and happier!

Graphs

• Happiness vs Income by Country:
  

From Lord Richard Layard's website: (http://cep.lse.ac.uk/layard/ )