Comment by wallflower

I've heard someone explain to me that it's kind of like a flag fluttering in the wind. Except that your tongue is the flag. He recommended that I try to stick the tip of my tongue to the roof of my mouth and blow out hard - and resist the outward pressure (and the tongue will flutter back rapidly like a flag - the Spanish R vibration) - and once I got that, with time, the amount of pressure needed would relax.