First I wanted to write that, speaking American English, you MUST NEVER use your tongue. No, that would be too bad...
I think Americans are less attentive (/accurate /thorough) to their pronunciation (/articulation /speaking) than the British, whatever dialect they (the British) speak. It might help to close your nostrils (/nose) with your fingers when speaking. At least when you're alone. And work with your lower jaw. Move it more than necessary (/required /essential).
If we ever meet do me a favor and don't use the nose-and-jaw-trick. I would understand you much better.
D., who always needs a few minutes to 'tune in' when speaking english.