Here are some useful sites for resources and reasearch:
Alexander Technique helps you develop a positive head-neck-back relationship which aids freedom of the voice.
This site is a great resource for pronunciation exercises, vocabulary, and many other tips of speaking English as a second language.
This site has a catalogue of videos of speakers around the world speaking texts in English. They focus on the mouth of the speaker so that you can see exactly what is going on in order to reproduce that accent.