[csvose]

Try teaching the ah sound. Then as your students get better tell them to add the rrr. Such as ahhrrrrrr. Also, I found that in saying the letter r the tongue curls up at the tip but generally doesn't touch the top of the mouth like it would for d or t sounds. It's important for the student to understand that in making the r sound the tongue makes a rolling motion forward and then back in the mouth. Hope those ideas help.