Twelve tips for acting like a true friend.

Everyone from Aristotle to Martin Seligman agrees that friendship is one of the keys to happiness.

No one would argue that they DON'T want strong friendships, but the trick is figuring out how, exactly, to keep your friendships strong.

Here twelve tips for how to act like a true friend:

1. Be supportive when your friend has bad news. This is perhaps the most critical duty of a friend.

2. Be supportive when your friend has good news. This is trickier; surprisingly, it's sometimes harder to be supportive when someone gets a promotion, gets engaged, or enjoys other good fortune, than it is to be supportive when someone is going through a hard time.

3. Don't gossip. It's not nice. Also, although it may be fun to gossip about Pat with Jean, Jean is probably going to feel wary of being your friend-you're not trustworthy. Along the same lines…

4. Keep a secret. One of the most satisfying aspects of friendship is that it allows two people to confide in each other. Spilling secrets will destroy that. Ah, it's so delicious to disclose a secret-but you have to resist.

5. Exchange favors. Along with the feeling of intimacy, one of the best parts of friendship is the feeling of support it provides. And while getting support is important, giving support may be even more important for boosting happiness.

6. Don't criticize a friend's sweetheart or spouse-and, at the other extreme, don't flirt with a friend's sweetheart or spouse.

7. Be kind to a friend's children.

8. Be friendly to a friend's friends. In fact, in a phenomenon called "triadic closure," people tend to befriend the friends of their friends - and this is very satisfying. Friendships thrive on inter-connection, and it's both energizing and comforting to feel that you're building not just friendships, but a social network.

9. Show up. Sometimes a friend wants you to show up someplace when you'd really rather not: a wedding in Topeka; a surprise party that falls on New Year's Eve, when you'd rather be doing something else. Recognize a command performance, and don't miss it.

Bonus activities:

10. Remember birthdays.

11. Be nice to their pets.

12. Help a friend think big. Nothing is more encouraging than a friend throwing out some huge goal and saying, "You should do that!" "You should write a book, you should start your own firm, you should run for office, you should join the Council on Foreign Relations." You never know, sometimes one encouraging comment can have extraordinary effect on someone's life.

Have I overlooked anything?

* Interested in starting your own happiness project? If you'd like to take a look at my personal Resolutions Chart, for inspiration, just email me at grubin, then the "at" sign, then gretchenrubin dot com. (Sorry about writing it in that roundabout way; I'm trying to thwart spammers.) Just write "Resolutions Chart" in the subject line.