I am a fan of Scott Adams's (writer of Dilbert) blog posts on humor. Here's his post on his humor formula. Excerpt:
The core of humor is what I call the 2-of-6 rule. In order for something to be funny, you need at least two of the following elements:
Cute (as in kids and animals)
Naughty
Bizarre
Clever
Recognizable (You’ve been there)
Cruel
I invented this rule, but you can check for yourself that whenever something is funny it follows the rule. And when something isn’t, it doesn’t.
And here's his post on humor writing, with reference to this post. (These links are all to the Internet Wayback Machine because he's moved his blog and the original posts are now offline. You might find the HTML a little off, so if you don't see the content of the blog post, just scroll down a bit.)
Reading the recent list of rationality quotes arranged by karma underlines the popularity of funniness, and being funny should probably be included in the pursuit of awesomeness.
My best guesses about characteristics of humor: If there's a word which makes the line funny, put it at the end. Phyllis Diller recommends that the word should end with a hard consonant (t or k).
If you can make a surprising statement extremely concise, there's a reasonable chance it will be funny especially if it includes an insult about an acceptable target.
Quasi-quote from Jim Davis, author of Garfield: "If I can't think of anything funny, I have one of the characters hit another." Any other principles of humor and/or methods for cultivating the ability to be funny?
ETA: The most recent thing that struck me as very funny-- how does it fit into the theories?