I think one problem that a lot of people have when trying to do accents is that they find one or two "hooks" and overplay them, while forgetting all of the subtle things that make an accent work.

A good example is that any time an American tries to "do" Canadian, he keeps throwing "oot" and "aboot" into every sentence.  Not all Canadians have that “Canadian raising“, and those for those that do, it's much subtler than it usually gets done--and the different vowel pronunciations get totally missed.Back in my radio days, I was the station’s “accent guy”—I probably wasn’t the greatest in the world, but I tried to make them as close as I could get.  I know I was the only guy in the area who was doing a “northern England” accent.