Lynn Bo is the founder of Accent Master. She is an American teacher who has an interesting blog that has videos and podcasts as well as posts regarding accent reduction or accent modification. You may find them useful, if you are interested in studying or imitating pure American English.

http://accentmaster.blogspot.com/

I have a slightly different accent, as I am from Toronto, Canada. My vowels are tighter and my enunciation is clearer than the average American. This is not a judgment, it is a fact, just as the British English accent is even tighter than the Canadian. That is how to tell the difference, and to determine where a person you are speaking to comes from.

I teach Accent Reduction a lot here in Toronto. I enjoy helping people speak clearer and more confidently. I know it is hard work to ‘teach an old dog new tricks’, but it is worth the time if you feel your accent is holding you back from personal or professional success.

Once again, here is Lynn Bo’s blog to check out:
http://accentmaster.blogspot.com/