A good checkpoint when you're at the top of your swing is that your left forearm (for lefties) is in a vertical position with the left elbow pointing down to the ground. Having the arm in this position (see photo) eliminates the 'flying elbow' that so many amateurs have. The flying elbow will make you 'come over the top' during your downswing and result in the 'outside-in' swing path that causes the slice. I'll admit it's tough to get in this correct position but it's something that you want to work on to improve your ball striking.
