I've said it a million times here before - it's VERY easy for us to do an American accent because of our familiarity with it. From an early age we are bombarded with it in a barrage of tv, sitcoms, soap operas and movies.
You just lengthen your "r"s("carrrrrr", not "cah" like an Australian would say)and soften all "a"s when you would normally do the opposite("dance"[rhymes with "prance], instead of the English "dahnce")and you're good to go.
Just for the love of GOD, please, please, PLEASE never pronounce "pecan" as "pee-KHAN", the way Kate Mulgrew does, or I might have to come around to your house and rip your tongue out with my own hands.
That is just.................FUCK.