The number one cause of slicing, irrespective of the exact aspect of the swing that is lacking, is a "grip it and rip it" mentality to your swing. This generally causes a number of timing and posture issues that open the clubface, bring the swing out of line, etc etc. So, the number one thing to do is slow it down. You can send the ball MUCH farther than you think with a very easy swing, and an extreme windup (the kind where you end up able to see the clubhead out in front of you in your peripheral vision) will cause more problems with accuracy than it solves with distance.

From a technical standpoint, slices happen when the club induces a clockwise (for a rightie) sidespin on the ball. This induces a force called Magnus lift, where the leading edge of the rotation on the left side of the ball brings the turbulent air from the back of the ball around to the front along that side as part of the ball's "boundary layer", and so causes the boundary layer on that side to be thicker. That thicker, relatively higher-pressure boundary layer pushes the ball in the opposite direction (to the right).

Sidespin is induced whenever the clubhead moves in a direction other than the direction the clubhead faces. The direction of the face determines the line the ball will launch along; the difference between that line and the line of movement of the clubhead determines sidespin. For a slice, the clubhead is moving to the left of the direction it faces. Most commonly, this is because the clubhead faces right of the intended line.

Here's how to diagnose and fix the most common slices, for a right-hand golfer:

* **Push Slice:**
 * **Symptom** - the shot launches on a line off to the right of the intended line, then curves even further right. The ball may also launch high.
 * **Method of Diagnosis** - Rotate the club counterclockwise by a few degrees at address to "close" it, until the launch angle or the flight path is straight. If the shot goes down the intended line, the problem is an open clubface with a straight swing. If the shot starts straight but still curves right (a "fade"), the problem is an outside-in swing path inducing spin, coupled with an open clubface. If the shot starts right and flies straight (a "push"), the problem is an inside-out swing path coupled with a very open clubface. If at any point you feel the club twist in your hands, you're hitting the ball with the toe of the club.

* **Push:**
 * **Symptom** - The shot launches right, but flies straight.
 * **Method of Diagnosis** - Close the clubface. If the ball starts straight and curves left (a "draw" or "straight hook"), the problem is an inside-out swing coupled with a slightly open clubface that squares the face to the swing path.

* **Fade/Straight Slice**
 * **Symptom** - The shot launches straight or slightly left, then curves right in the air and may climb.
 * **Method of Diagnosis** - This is almost always an outside-in swing path with a square clubface. To confirm, close the clubface by rotating the shaft counter-clockwise at address. If the shot launches left and flies straight (a "pull") or still drifts right (a "pull slice") this is the problem.

* **Pull Slice**
 * **Symptom** - The shot launches left and possibly low, but then climbs and curves right.
 * **Method of diagnosis** - Open the clubface; if you get a straight slice, the problem is an outside-in swing, which you're attempting to compensate for by closing the face slightly (not enough to square it to the clubhead path, which would probably pull the ball hard left). 

* **Solutions:** 
      * All around, you're probably "overswinging" the club. This causes breakdowns in key areas of posture and mechanics, and can cause the clubhead to torque causing changes in face angle. Slow down the backswing, and don't take as big a backswing as you normally do. Don't "break" your wrist on the backswing; that puts your hands too far in front of the clubhead and opens the clubface. You might choke down on the grip an inch, maybe two, and concentrate in swinging through a level plane to hit the ball over swinging the club as hard as you can. You can play a 200-yard drive that lands right down the middle of the fairway; a drive that might have gone 300 yards, but instead took that huge right turn out of bounds, is near-useless.
      * To solve toe hits, get an inch closer to the ball as you take your stance, and make sure the ball lines up with the instep of your left foot. Visualize swinging "out" at the ball a little more than "down" at it (but be careful not to "top" or "whiff" the ball by swinging over it). You may benefit from a longer shaft, or to have the lie angle adjusted (if possible).
      * To solve an open clubface at impact, first ensure the ball is in line with the instep of your left foot. If the shot launches high, try moving the ball a bit forward; if the shot launches low, bring it back a bit. Try switching from a grip style with more solid right-hand placement to one that overlaps more. If you use a baseball grip, try an interlocking; if you use an interlocking try a Vardon Overlap. Be mindful of your right shoulder; it's tempting to "drop" that shoulder to make the motion more like swinging downward (which uses more core and adds power) but that will cause your hands to twist. Lastly, be mindful of where your hands are at impact; the further forward they are, the more open the clubface will be. Try releasing your wrists earlier in the downswing or more suddenly as you start to bottom out, to induce more of a "whip" through the ball.
      * A swing path problem is generally solved by making sure you address the ball properly, and then "flattening" the swing path so that the clubhead moves through one plane of motion.
         * For ball address, in all cases the ball should be placed inline with the instep of the left foot. To determine how close or far away the ball should be, stand with your feet a little more that shoulder-width apart (it should be comfortable to stand upright that way, and you should still be able to twist very easily at the hips), and bend at the waist without sticking your butt out until you're looking at the tips of your toes and your weight is on the balls of your feet. Square your shoulders with your feet, and let your hands hang naturally. Now, place the club in your hands and ground the clubhead on your center line. Keep your hands hanging naturally; the shaft should be pointing right up to about your belt buckle. The ball should be lined up just a *smidge* inside the current line of the center of the clubface (to compensate for the fact that it's further forward in your stance than the clubhead should be at its bottom point)
         * For swing mechanics, an inside-out swing with proper ball address is caused by swinging down, then out; you're trying to hit the club too much like an iron and then having to correct mid-swing to fit the longer shaft in between you and the ball. Fix it by lowering your backswing a touch and visualize starting your swing more "out" at the ball. An outside-in swing is caused by swinging out, then down. You're trying to differentiate the driver from an iron too much, going for a "power swing" baseball style, and are correcting mid-swing to make contact. Fix it by raising the backswing a touch to bring the downswing into the same plane, reducing your need to twist your body around or pull your hands in to make solid contact.
      * As a short-term solution to most of this, rotate the club counterclockwise to close the face until the shot flies straight, and correct your aim left or right to compensate for the swing path.