^ What he said. It sounds so cliched and irritating, but it is so very true. I know "Practice!!" was the last thing I wanted to hear when I was learning, but in hndsight, I wish I'd practiced more.
Like any other technique, practice it ridiculously slow, with a metronome. Slow enough that your brain has the time to comprehend coordinating both hands the way you want them. Once you get that down slow, gradually increase the speed a few notches, to where any faster and it'll get away from you. Master it. Increase. Repeat until you're where you want to be.