As a British ex-pat now living in the States, I've had no end of fun making myself understood by regular Americans.
The first day I was here, my wife took me to an Applebee restaurant. When it came to ordering the drinks, the conversation went something like:
Server: Can I get you guys any drinks?
Wife: Yes, I'll have a Diet Coke.
Server: Okay.
Me: And I'll have a bottle of Budweiser.
Server: Huh?
Me: A bottle of Budweiser?
Server: Erm...
Wife: A bottle of Budweiser!
Server: Oh! Hey, you've got a weird accent!
Me: Thanks!
Americans are typically expecting to hear "BO'LL". Of course, I was pronouncing it as "BOT-ALL", which confused the poor server no end.
I've been living in the States for over a year now. My mother-in-law and sister-in-law swear my English accent is softening, but I think it's more a case of them being used to me now. I generally find that people have less problem understanding me if I raise my voice several decibels above the norm (British people tend to be, on average, more softly-spoken than the average American) and/or if I enunciate with some deliberation.
That hasn't stopped at least one person thinking I'm Australian though.