I learned how to do this at 14 (which is about a year ago or something). It's worth the effort if you really want to speak Russian (or any language with this sound) well. You need to practice to get it, and once you do it's the easiest thing ever. First of all, a rolled "R" sounds like a lazy "d" or "t" in English. Listen to "Red Hot Chili Peppers: Give it Away" and you'll hear it! Say words like "butter" (pronounce the T's as soft D's) & "ladder" quickly and you may notice a short rolled "R", I did. And remember, as Superbee said, a tongue trill is a vibrating tongue with rushing air causing the impact.

These videos helped me learn:

<a href="http://www.youtube.com/watch?v=P9mMvuRGKY8">http://www.youtube.com/watch?v=P9mMvuRGKY8</a>;

<a href="http://www.youtube.com/watch?v=9gOZTXqgKcY&feature=youtu.be">http://www.youtube.com/watch?v=9gOZTXqgKcY&feature=youtu.be</a> ;(if tongue is stiff) 

all the secret is that your tongue need to vibrate. I knew one exercise it's used with lisping children. you place the tip of the tongue near the teeth and try to make it vibrate with а thud sound "trrr", you need long saund, so vibration will tick you nose. only lot of practice can make an effect. I practiced about 2 months cause I've some difficulties in childhood. After you catch tongue position you can easily say short "r"

sounds like "r" in "rest" but <a style="color: #000080; font-family: Verdana, Tahoma, Arial, sans-serif; font-size: 12px; line-height: 17px;" href="http://masterrussian.net/f13/rolling-%D1%80-r-sound-important-not-18556/" rel="nofollow" target="_blank">rolled</a>.

Try not to touch the tongue to the teeth