I can do a great Scotty (from Star Trek), but I can't do a good Demoman, for whatever reason. I can nail the "I'm givin' her all she's got, captain, I don't think she can take it any longer!" but I screw up on the "What makes me a good Demoman?".
I usually practice some accents while I'm driving, but I've never actually had a problem where I couldn't go back to a normal accent.
I can't do eastern european accents at all. For some reason, I just can't get it to sound right no matter what.
My normal accent isn't really tied to any specific place, since I've moved from place to place even when I was a child. I never really picked up any local accents because my family was never anywhere long enough to do so. So it's kind of a conglomerate of Western accents.