Tense mouths

The Japanese mouth and jaw is pretty tense compared to the American mouth and jaw. This is because 4 of the 5 Japanese vowels are tense （あ is relaxed).

Rebecca Linquist has a good video explaining the basic problem and what to do:

I do recommend Rebecca's video lessons for accent reduction. She's helped quite a of my current and former students improve their accent. You can learn more about her program at the English by the Hour homepage.