Just set yourself a route/section and ride from one side of a pile of rocks to the other. You'll soon learn the range of moves you need to complete the section, then you can concentrate on them.
Myself and everyone I ride with takes Glucosamine tablets to help out our knees... They're all f**ked. We look like retards when we go to the clubs/try and walk.
You gotta not really kick at all, just use pedal pressure, compress down into the object as you get alot of tyre compression, then spring up without kicking much.
More of a knack than anything.