Its something that is done unconciously. At least in my case. We tend to mime or imitate naturally. When I travel and have been in some place a few days, it is not at all strange for people to tell me I have no Texas accent.
Mine definitely lessens too. When I was in the Navy, people would poke fun at me for days after I returned from being home on leave because my accent would return to its normal level. My post was really in reference to people doing it intentionally though. I would use my mother-in-law as an example again but that wouldn't be nice.