After a year, people I meet tell me I sound very "token" American. So I guess I haven't started to neutralize yet- though when I talk to Brits I try to use the British words (courgette, crisps, etc).
People at my Dh's office however, say that HE has a neutralized accent, and always pick on him when he accidentally uses the American word for things (soda, dude, fries and awesome especially).
We have an american family friend in our town that has lived here for 20 years, with no trace of a British accent, and an in-law that has been here for 40 years and still sounds American. Both of our friends that came over when they were children ( 5 & 10ish) have both lost their american accents though, so like the previous posters said- it may have to do with age/impressionability.