I have a problem pronouncing R'. I'm Indian and this mispronunciation makes my accent seem American to people. What my speech therapist recommended was to roll my tongue when I said R and in order to practice that he gave me an exercise. You know how a motor sounds? RRRR? Or more like TRRRR? SO, I do that to improve and it has helped a lot! Before when I said Rabbit, it sounded like VABBIT, but now I can say it a lot more clearly. Hope this helps!