For right-handed golfers (sorry, Bubba), here is the science behind the slice. Ready? The clubface is pointing right of the direction the club is moving as it strikes the ball. That's it. If the path is straight at the target or moving a little in-to-out in relation to the target, the ball will start right of the target and curve even farther right. If the club's path is out-to-in in relation to the target line, the ball will start left of the target but curve back to the right. That's why you often see slicers adjusting their bodies more and more to the left of the target in the hope that they start the ball far enough left that it won't overshoot the target on the right. Unfortunately, the more they swing out-to-in, the more the ball curves.
There are a lot of things you can do in the golf swing to prevent a slice, but if you want to straighten it out for good, and even start hitting draws, you have to train your body to let the club swing down from the inside. The most common reason amateurs slice the ball is because their bodies block the club from swinging in-to-out. They have to move the club from out-to-in just to get it back to the ball.
Remember that during the downswing, your trunk should rotate toward the target, but the shoulders should not stay level. They need to rotate while you remain in the spine tilt you created when you addressed the ball. That means your right shoulder should move down toward the ball while your left shoulder moves up and away from the target. Most amateurs lack the lower-body stability and mid-back mobility to make this happen. But there is one great exercise you can do in the gym to train your quadriceps, hamstrings, glutes, abs and obliques to generate proper trunk rotation. You can even do it as a warm-up before you play. A weighted bar is ideal for doing this exercise, but you can use a golf club or similar. Click on the video below to watch a demonstration.