There were a few good tips in this thread -
http://yoyoexpert.com/forums/index.php/topic,79532.0.html
Basically - Pull straight up, use knees to help and lower the ken to the tama, don’t just try to pull the tama straight up to the ken. I find it helps if you use your whole arm to pull, lift your shoulder a little and don’t flick your wrist or bend the elbow too much, that will jerk the string and lead to the ball turning.
Keep on practicing and you’ll get it down eventually.