It's hard to really articulate what the parameters are that make one song parody-able and another song not, but if I can come up with a good enough idea for it, I go for it, and if not, then I have to move on.
Somebody will come up to me after a show and have me sign their arm, and the next time I see them my autograph has been permanently inscribed on their arm.
One of my pet peeves is that sometimes the talents of my band get overlooked because, and it was the same problem that Frank Zappa had, with a lot of groups that use humor, people don't realize there's a lot of craft behind the comedy.