Here is a free online talking dictionary http://howjsay.com/

Speaking to native speakers, and asking them to correct your pronunciation. Although online (skype/qq) helps, the best thing is live, as they can help you with your 'mouth shape' for tricky sounds.

One of the ways is to listen to an English audiobook narrated by professionals.

Elsie: 1. I once knew an outstanding teacher at the best American ESL school for adults (in my opinion). Her students loved her because she taught them exactly where to put the tongue in order to properly pronounce American English. 2. She wrote a book called "Realistically Speaking." It also comes with a video and DVD. 3. I can highly recommend it.