just practice... lol sounds lame but i sucked at it really bad at first. Then one weekend I tried again after like a year of not really messing with it. and it clicked...
however my suggestion to learn/practice is take an easy trail but long nile mile, paradise, etc... just drop in switch, go slow, DO NOT DO NOT switch stances. you'll look like a newb at first which i hated the most really for learning but it's gotta happen... do it a few times and eventually you'll be able to translate your mechanics from being able to ride regular to switch. shouldn't take as long as getting good to begin with