I used this book with my son who was having so much trouble pronouncing the "R" sound:
http://www.amazon.com/Help-Me-Talk-Right-Lessons/dp/0963542613
When my son's difficulty in speech was interfering with his ability to spell words correctly, I decided to give this book a try before contacting the ps. I purchased mine from Amazon. I'm no speech therapist, but the exercises given in the book are easy to follow and now my son speaks well. We made speech part of our school day and he actually thought it was fun.