Yeah, repitition, repitition, repitition!
Also stretches would help you with your off-leg reach and felxibilty a hell of alot, groin and hamstrings stretches are bloody good.
And maybe even pretend you dont have anything below your knee. it'll get your thighs moving higher and higher, just imagine you're kicking the bag/opponent with just your thigh, and try get perfect balance there, and then picture it with your whole leg and bam! perfect rounhouse, diagonal, or straight kick.