Watch copious amounts of Fresh Prince of Bel-Air episodes. Haha!
Seriously, I would encourage you to start just saying the first thing that comes to mind for a while. Don't edit, don't censor (have some cooth and only do this when sarging, not at work, or with family and friends). Literally, doesn't matter if it's PC or even on topic. You think something, say something. Honestly, not sure if that will work. But worth a shot at least a couple of times. What do some other guys think?
I'm not sure about exercises. I think it may be one of those things you either have or don't have. Which doesn't mean a thing. Work on your strengths. Have some standard cocky/funny responses and such that can be applied to a range of situations, and try using them in those cases.
Let me know if you try either of these, and how it worked, if it did.