Normally when doing a RM, when you first jump up onto the backfacing pedal, any weight/back pressure will stop the wheel - momentarily - until you get up just past vertical and begin pedaling with the second foot which hits the forward pedal right after the first foot. The key is to allow the weight and momentum of the moving wheel to push your mounting foot up, along with the rotation of the wheel.

[ame="http://www.youtube.com/watch?v=JaSY5If3lUU"]Non-Stop Rolling Mount on a 36er unicycle - How to - YouTube[/ame]