I actually didn't learn to roll the "rrs" until well after my mission. I was working on campus and running errands all over and humming under my breath. Anyway, what started out as "pah-dah" said over and over very rapidly eventually became "pa-rra". I eventually figured out how to control the roll with other vowels. I still practice them all from "ar-ra, ar-re, ar-ri" up to "ur-ra, ur-re, ur-ri, etc." I hope it helps; good luck!