Lisa Chesser has built an impressive and useful list of ways to incorporate (engender) laughter in the classroom. Her list focuses on K-12, but it's easy to imagine most of these also in use at the college level.
I do some of them myself (self-deprecation, comic strips, music) but there are others in here I've never thought of and will definitely use. I love the idea of using tongue twisters as a quick break from the content and re-set of the attention clock!
See her list here: