I've been thinking about rolling your R's, and how my friend taught me to do it.
Here's my advice:
1. place the tip of your tongue at the top of your mouth. I'm not sure whether it matters where, it didn't seem to for me.
2. blow air out, if your tongue is at the roof of your mouth, it should hit your tongue and not escape your mouth.
3. the more you practice doing this, you should start to notice that your tongue will move, it'll start to flip a little, and then start to roll.
4. soon you should be able to make a noise that sounds like a motocycle revving...

You might want to practise away from other people, because apparently the sound gets very annoying after a while.
5. if you practice flipping your tongue whilst saying words with 'r' in them, you should be able to sound like you're rolling your 'r's'. If you practice even more, you might even be able to do it properly, unlike me, who is too lazy.
I managed to get to step 4 in the space of 3 days, surprisingly enough. It was very exciting!!! My brother thinks that the rolled r sound is something that you learn when you're very young, (for non r rolling language people) when you're experimenting with sounds... maybe that's why i never learnt, when i was young i had a speech disability and couldn't say the letter r!!!
Anyhow, hope this helps.

I cant garantee that it'll work though...