Increase your awareness and warm up your face, lips, mouth and tongue by reading from an English book in the following way: read out loud very slowly while greatly exaggerating the way your face, lips, mouth and tongue move to make those sounds. Then follow it up by practicing the "la la la" sound again and again for some extra tongue movement practice. Notice how the position of your face, lips, mouth and tongue changed to create each combination of letters when you were slowly reading out loud.
Practice your pronunciation of the Spanish alphabet by reading out loud from a Spanish book. If you do not already know the Spanish alphabet, listen to the Spanish alphabet audio files available on one of the many Spanish learning websites.
Open up the Spanish dictionary to the R section. Pick a few short words that start in r, as these will be the words you will practice on.
Practice the r words while aiming to curl your tongue toward the top of your mouth. At the same time, push air out from your lungs, over your tongue and out through your mouth as you pronounce the rr, letting your vocal chords vibrate. The vibration will be similar to a purring sound. Practice this again and again. While at first it may seem complicated to coordinate the tongue movement, flow of air and sound vibration, once you've achieved and practiced it, rolling your r's can become second nature.