I'm rather fond of Benign Violation Theory, as interpreted by Robin Hanson. Basically, funny things are those that benefit people you're sympathetic with (in particular, raising their social status) by violating existing social norms.
I find this doubtful. I've attended a speech by Ann Coulter, and found some of her lines legitimately funny despite the fact that they were attacks on the status of groups I'm sympathetic to. This doesn't just fail to fit the model, it seems flat out contradictory of it.
The norms that are thus violated are also norms of your in-group, otherwise they wouldn't be considered norms.
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?