Most common causes of a slice is pulling in towards your left hip as your come through the ball. This causes side the face to move/rotate open on impact. Creating a side spin as it comes off of the clubface. The placement of your eyes is a major factor as well. Looking at the side of the ball, can cause you to pull towards the target, thus causing the over the top motion, creating the side spin.
One drill I like to tech my students not to pull over the top is to take the club up full to your backswing, then drop the club behind you. Finish your full swing forward while the club drops. This teaches your muscles to drop down instead of push outwards. Allowing your to swing full through without any pull in your shot. Repeat this till you can feel the drop in your swing before your turn starts.
Another way to think of it, is to picture a low outside fastball that yo want to hit to right center field. Swing with a slight outwards path to help keep that face closed through the ball. If you are coming over the top, there is no way to bring that club anywhere but towards your body. Hug your right elbow closer to your side, and allow yourself to swing on an outward path.