The drill Ben suggested is a good one. However, I would recommend something lower in height. Considering you're working on correcting a motor pattern, the height of 20-kg plates will be somewhat unusual to you and instead of just focusing on staying on your feet longer, you're going to start thinking about other things since the increased height changes the way you pull. I recommend a simple piece of 3/4" horse stall mat to make it as close to normal as possible during the toes-off drill.
Another drill you can do is a snatch deadlift. Even better, do a halting snatch deadlift to the hip to beat in how high the bar can travel before you transition into the second pull.
After you feel the difference, incorporate a snatch pull. You can do all this on that piece of mat, then transition onto the floor, especially once you want to start to snatch. You have to remember that you want to PUSH against that ground as long as possible.
Is this just a problem in snatching, or does it surface when cleaning as well? I'm of the believe that if you correct motor pattern in the snatch, then it normally transfers to the clean as well.
I hope this helps!
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.