There are many reasons that golfers slice the ball but in my years of coaching experience, one of the main culprits is the way the body moves during the downswing. Instead of keeping your upper body nicely centred during the swing, players let their shoulders move ahead of the ball at impact.
This creates too steep a downswing and the result is an ugly left to right ball flight. The answer is to turn your shoulders to stop slicing.
Ensure your angle of attack is not too steep by making a full shoulder turn. At the top of the backswing your back should be facing the target.
Of course, not all of us are able to rotate as far as Rory McIlroy but you should make as much of a turn as you can. If you hold the position at the top of the backswing it SHOULD feel uncomfortable. This is because you have created resistance between your upper and lower body and this is responsible for creating power in the golf swing.
To help you cure your slice with shoulder turn imagine a ball teed at shin height. Your shoulders are forced to turn and your swing will automatically find a flatter path. Your upper body will not lean into the ball but turn powerfully, coiling behind the ball ready for a rapid rotation back to impact. Moreover, you will also activate a better release of the golf club through impact.
Your hands should naturally rotate through the ball, helping to square the golf club at the key moment.
This is a great drill to help you fix your slice. A simple visualisation of a ball teed up two or three feet will activate your shoulder turn and allow you to make a solid, powerful contact with the golf ball.