Voice quality is one of the most important aspects of the accent -- it's the essence of the American sound. You want to speak from your chest and project your voice out clearly. This will not only make you easier to understand, but it will make you sound more confident. If you think your voice is nasal, test it by holding your nose closed and saying baby or hot or fast (or any word without M, N or NG). If your head vibrates, you are speaking nasally.
Keeping your nostrils pinched together, say a sentence with no nasals in it, such as Joe eats a lot of fish. If there's any vibration at all, that's the indicator of nasality. You then need to drop your voice down to your chest. You will hear the difference immediately, and you will physically feel the shift.