high pulls will help strengthen the initial pull down to the bar, although don't use them if you have trouble over pulling in the lifts, i.e. dont change directions quickly.
tall pull-downs are another option - stand tall and simultaneously move the feet and squat partially as you pull down against the bar. basically this is a high-pull down or Chinese pull without accelerating the bar upward first.
muscle cleans/snatches will help train the mechanics of the pull under/turnover and strengthen it a bit. just make sure you're getting max elbow high and a very aggressive pull of the elbows up before you start the turnover.
finally, doing snatches and cleans from the high hang or high blocks will force you to be more aggressive changing directions and pulling under.