The biggest reason for 'skying' the ball is when you treat the Driver too much like an iron shot. With an iron shot you want to feel like you're hitting down on the ball so that the ball goes up. With the driver however you want to feel as though you're hitting up on the ball. When I say up on the ball, I still though want you to feel that you're still transferring your weight through the ball.
The most important way to ensure you're hitting up on the ball is through set-up. Ensure with a driver that you move the ball opposite your left heel for a right hander. Also you want to make sure you tilt your right shoulder down more and feel as though there's approximately 60 percent of weight on the back foot. When you make all these adjustments and swing through normally, you will feel as though you're hitting more up on the ball.
If you're practicing on a mat with a rubber tee protruding through, ensure that you're practice swings are hitting the tee and not the mat. Also when placing the ball on the tee, ensure the tee height is correct. The rule of thumb is that half the ball should be above the top of the club. You could even go lower at the start to really ensure that you don't get under the ball.
There's also a great drill that you can use including placing your driver head cover around 2 driver head widths in front of the ball and try and hit through without hitting the cover. Once you have achieved this, you will have successfully hit up on the ball and thus eliminating the sky shots.
If you would like to book a lesson to work on hour driver just let me know.