One of the most common causes of the slice is an outside-to-inside swing path created when golfers raise their hands at the start of the downswing, coming down on a steeper plane (coming "over the top"). An outside-to-inside path causes the face of the club to cut across the ball, imparting slice-inducing spin.
Most of the best golfers in the world approach the ball on an inside-to-outside path.
The "Up the Hill" drill can help recreational golfers learn what an inside-to-outside swing path feels like. Here's how it works:
As shown in the photo above, take a stance at the base of a slight incline. The lines on the grass represent the path that you will feel the club is taking. The club is not actually traveling straight (white line), but it feels that way on a normal takeaway and follow-through.
To make yourself swing on an inside-to-outside swing path, try to swing "up the hill" (as if you are following the red line).