The hardest thing about pronouncing Spanish for me has always been rolling my r's. For years I was convinced I'd never be able to do it. Now I've got a respectable (if not always reliable) rr, so I thought I'd share my tips for others who have struggled with this maneuver.
First, you must be able to "flip" your r's. I believe this is something every native English speaker can do, and it's a reasonable fake until you master the roll. It is something like a very quickly spoken d between two vowels; say "yadda yadda yadda" fast and you'll produce it. This is what the single r sounds like in Spanish, as in the word cara. Never pronounce the r as it is done in English. This will mark you as a gringo faster than you can say "car-uh."
Now, the rr is theoretically just this sound repeated, but that's much easier said than done. One great discovery for me was that the rr is much more easily pronounced after certain vowels. Unfortunately, most Spanish students encounter the sound first in the word perro, when I find it much easier to produce the rr after the vowels a and o. When I feel myself "losing" the roll, I say ahorro or gitarra several times and it almost always comes back. Start by using the flip, then slow down the word until the flip turns into a roll. It won't come immediately, but it will happen. I am living proof.