There's an entire page in the "TRICKS YOU NEED HELP WITH" thread already, but to try and help you everyone always told me to try and tuck your feet up. If you look at pictures of people doing them look at their feet, they have them up so the deck can come around before their feet hit the ground.
Provided, I catch them all with one foot but for some reason I can't get my other on, but that's just me. I found that just practicing the motion over and over and over again, going as fast as your arms will let you makes it go around so fast that it will just get underneath you anyway.