Singing along to French songs in the car finally fixed my R problem, in a matter of days. Found I could do the rolled Spanish R and even the lazy, liquid Latin R with very little practice after getting that one down, too, and I couldn't at all before.
Do you mean "de rien"? (As in "no problem", literally "it was nothing")
I agree r's are definitely the hardest thing to pronounce in french (as an english speaker at least). I think "rire" is the hardest word in the french language