How to write with humor

Gwen Bristol asked me to write about how I add comedy to my writing. Well, honestly, I am not sure. Most of my books lean more to the serious side. However, I have found that sarcasm is the easiest way to make my readers laugh. My main characters are usually lacking in self-confidence, so they always poke fun at themselves. I like my characters to be humble, even thought they have great responsibility and are awesome in action. That seems to always end with some sort of humor, no matter how dry.

I really liked this question because it made me think. But tell me something: how do you add humor to your books?