When I moved to the states nearly 20 years ago, my pronunciation sharpened up (stopped dropping consonants) very quickly. Over time I stopped using idioms, and the few bits of Cockney Rhyming slang I used (Butchers, Scooby; which are look, and clue, respectively).

I think it was less as a conscious act and more as a result of just not being around people that use them. There's a sizeable element of cultural reinforcement involved.

That said, they'll pry my British spelling out of my cold, dead, hands.