I agree with Damien. But I think it's easier for an American to roll an R than for Chinese. I can do the Spanish Rs pretty easily, but have a tough time with Chinese.

hmm it's natural that you can't pronounce it without alot of practising.. with my language (dutch) you need the rolling R too, and alot of chinese people live here.. who can't pronounce it well, even after 10 years of living here. so don't worry to much about it, we will understand you anyway if you make a L of it :] but jia you~

A trick I learned a long time ago to *sound* like you're rolling your R's is to say a quick "D" sound. Now, I don't speak Spanish, but my Spanish-speaking friends say this is the easiest loophole. Instead of "Para llevar, por favor", say "Pada llevad, pod favod", keeping all the other pronunciation the same. It's really a simple trick, which will eventually lead into you rolling your R's. Another way to look at it is to pronounce a really soft R sound, right before a D sound. "Parda llevard pord favord." As you're starting to pronounce the R, switch into D sound. Your tongue will feel like it's clicking a little, at least that's how mine feels. Hope this helps.

Here's a list of methods that could help: http://www.wikihow.com/Roll-Your-%22R%22s