I have a great deal of difficultly properly rolling my r - how should a native English speaker learn this sound? Hopefully our resident Spanish teacher can help me out, as I would assume it is a fairly common problem.
<rolls up sleeves> OK...
Lemme see if I can 'splain this over the 'Net...
Place the tip of your tongue on the roof of your mouth, behind your top teeth, as if you were getting ready to make a "d" sound.
Relax your tongue slightly, but not SO much that you lose the "d-sound" shape. Really, the BACK of your tongue should be relaxed.
Lower the back of your tongue and force air past your tongue's tip and out your mouth. The idea is to force the tongue forward just enough with your breath to where it (the tip of your tongue) leaves the roof of the mouth and then immediately snaps back to its original place (since you should still be holding your tongue there), then out again, then back again, etc...the "rolled r" sound is actually this repetitive, staccato stopping of air by the tip of your tongue against the roof of your mouth.
Hope this helps.