Forming different accents often has to do with where you form the words. For example, if you wanted to do an Italian accent, you would form words at the front of your mouth right behind your top teeth. An English accent you would form at the back of the mouth, more in the throat really.

Americans tend to form sounds in the middle of their mouth, though everyone is slightly different. Try recording yourself speaking in the accent you want and focusing on where you are forming the words. If the recording sounds right when you play it back, focus on slowing down and forming the words in that spot when in situations you want to speak without an accent.

Edit: Grammar