Try this video http://www.youtube.com/watch?v=eLsUXDkVKYA&feature=player_embedded - it is created especially for English speaker and helped me a lot! :)

I've found that practicing A LOT helps tremendously. If you can get the 'r' to roll just for a short time, then focus on it. Every time I drove somewhere, instead of listening to music in the car I would just try to roll my 'r's constantly, haha! See if that works out for you.