The proper approach and body alignment are vital to curing a wicked slice. So, as you step up to the ball, make sure that your feet are shoulder width apart, ball in the middle of your feet. Check your feet to see that they draw a line from the big toe of your back foot past the big toe of your front foot and straight out to where you want the ball to travel.
Your arms should be extended slightly. Your knees should be bent. The ball should be easy to reach, but allow your body to extend once you begin your swing. Being comfortable in your stance is tantamount to success. So get in your stance and place your club head behind the ball. The club head should be straight on with the ball, not turned in or out but instead 90 degrees perpendicular to the ball.
Visualizing Railroad Tracks
The easiest way to make sure that your alignment and stance are correct is to imagine a set of railroad tracks running off in the distance to where you want the ball to travel. Your feet should be position on one of the tracks, the ball on the other. As you begin your backswing, keep your club head inside the outer track (the one your ball is on). If you can keep your club head inside this track and control your body, you'll lose any urge to drop your shoulder or turn your club head. Make sure that you stay inside the track on your backswing and follow through. Do this and your slice will magically disappear.