This website was created to help people improve their American English pronunciation. At the core is a series of videos explaining in detail how each sound is created, as well as a blog.
Take a look at how the mouth position changes on the different vowels:
Video Text:
[
] ('ah' as in 'father') ... [
] ('aw' as in 'law') ... [
] ('aa' as in 'bat') ... [
] ('eh' as in 'bed') ... [
] ('ih' as in 'sit') ... [
] ('ee' as in 'she') ... [
] ('oo' as in 'boo') ... [
] ('uh' as in 'pull') ... [
] ('uh' as in 'butter') ... [
] ('uh' as in 'supply') ... [
] ('er' as in 'her') [x3]