To truly roll your r, you need to forget the American r pronunciation. The American r is hard, formed with the lips closed, and with your tongue on the bottom of your mouth. The Spanish (or French, or other rolled r) is much more similar to the English "d" sound or even "l". The rolled r is pronounced with the mouth open, with the tongue just behind the front teeth.
Practice a few times saying the d sound aloud, then follow with the rolled r. Your tongue should not move very far. Your lower jaw should not change at all.
Now that you know how to form it, practice drawing it out very long. Take a deep breath, put your tongue on top of your mouth and make the ongoing rolled r, like a motorboat. This is great exercise for your tongue!
Now practice the real thing! Get out your Spanish dictionary and practice those words with an r in the middle. For advanced practice, try those words that start with an r. Another great place to find practice words is in the phone book of a major city. Find a hispanic name and go down the list carefully pronouncing the names and rolling your r.