I generally am not in situations that I need to make comebacks. Heck, the last time someone said something that I could have used a comeback, I just turned and began to stare him down. {: Hey, it worked, at least.
I'm good with sarcastic remarks, but I usually use them in the way of making people laugh. You really have to get on my bad side before I switch from amusing to insulting.
The closest I've come recently to a comeback was during a conversation with my older sister. She told me not to be proud that I'm taller than her (despite the large age difference). I replied, "I will be proud that I'm taller...not that it's much of an accomplishment."