Here are a few caricatures from photo references...It's easier for me to work from photographs because I can take the time to study the subject's features closely, instead of trying to quickly capture them as the subject is walking by. These particular photos were from a few caricature jams over at the Drawing Board.
Here's the reference pics
I think Uma Therman is my least favorite, and Matt Damon is kind of there...but I was fairly happy with how John Goodman came out. Below I posted my process to show that it takes a few tries to nail down a look I'm happy with.