Below are some websites that are helpful for those refining their pronunciation of American English:

This is a fabulous site!  It demonstrates, with animation and videos of real people, exactly how to say each of the sounds in American English.

http://www.uiowa.edu/~acadtech/phonetics/

Obama’s weekly speech is written out and each sentence is repeated three times.  It is a great learning/practicing tool.  Notice the movement of his mouth, his use of intonation (the pitch moving up and down), his rate, and his use of pauses. 

http://www.manythings.org/obama

Jokes

http://www.manythings.org/jokes/

If it is difficult for you to identify one vowel from another, practice vowel discrimination at this website.  Explore this website.  It has lots of information.

 http://www.manythings.org/repeat/

This site explains and discusses phonetics.  The International Phonetic Alphabet (which does not have to be memorized) allows us to use one letter or symbol representing each sound.

http://en.wikipedia.org/wiki/Phonetics

Native language phonetic inventories.  Look up your native language and compare it to the sounds that are used in American English.  You will see what your language has in common with English and which sounds will be new for you.

http://classweb.gmu.edu/accent/nl-ipa/inventories.html

This is an auditory IPA of sounds around the world.  If you click on the sounds you can hear them.

http://web.uvic.ca/ling/resources/ipa/charts/IPAlab/IPAlab.htm

Resources for studying American English.

http://faculty.washington.edu/dillon/PhonResources/