I have the best luck with really getting people to focus on their body position which translates to a good ski position for starts.
The simplest way I've been able to explain it is to get your hands down to your toes. This gets you to have arms straight and reached forward with your knees in to your chest (creates a low center of gravity and leveraged position for when the ski begins to plane).
The second key point is that your butt should be as close as possible to your rear binding. This pitches the ski more towards a plaining position.
As you are being pulled up, focus on continuing to push that handle down towards your feet by engaging your lats.
If you get these 2 correct, your driver should be able to maintain tension on the rope and get you moving a bit through the water before pulling you up. And as everyone wrote above, a smooth consistent push of the throttle is what is needed.