Quasi-quote from Jim Davis, author of Garfield: "If I can't think of anything funny, I have one of the characters hit another."
Well, yeah, but that's Garfield.
Which reminds me of an observation I've made: Consider that almost anything you can do to Garfield makes it funnier. (See: the Garfield Randomizer, Garkov, Garfield Minus Garfield, Garfield As Garfield, Lasagna Cat, Realfield, Square Root Of Minus Garfield, etc.) This does not work on things that are merely unfunny. In the same way that reversed stupidity is not intelligence, reversed unfunniness is not humour. This means that Garfield is not merely unfunny, but antifunny.
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?