There are a number of causes behind a skied drive but the underlying problem is having too much weight on your left side through impact.

Firstly, in the address position check that your ball position isn't too far back, your hands aren't too far forward and the clubface isn't closed. Now practice hitting a series of shots keeping 60% of your weight on your right side throughout.

This should shallow off your angle of attack helping you find the middle of the clubface once again.