One of the core factors to slicing is caused by a lack of turn on the backswing. The reason for this is two fold. The first being that if you don't tend to turn your body on the backswing, the body will tend to over rotate on the downswing causing the path to move on an extreme out - in swing path causing slicing.
The other factor has to do with the the overall path on the backswing. The less we turn our body on the backswing, the more the path gets steeper coming up and coming down into the ball. The steepness will cause 'fat' shots and a tendency for the path to work more on an out - in path.
What we nee to do is ensure that we turn our hips to around 45 degrees and our shoulder to 90. This will not only help with out power but also the swing path as the greater the turn, the more the path will swing around the body instead of steep.
With more rotation of the hips the path will be flatter and the body will be less likely to over rotate through the ball causing straighter shots and 'draws'.