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. At the driving range bay, determine the desired line of travel, and draw a line or place a club along that line. Put your toes right up to that line (a "square stance"), address the ball as you normally do, take your normal swing (ensuring you make solid contact), and find the symptom that best describes the ball's flight:

**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. You can confirm this on your driver by cleaning the clubface to remove any marks and dirt, making a few solid hits, and looking for the circular dimple markings left by the ball at impact. Or, use a strip or two of clubfitting tape (normally used to determine proper lie angle; it'll give you a nice big blue or black mark where the ball contacts the face)

**Push:**

* **Symptom** - The shot launches right, but flies straight.
* **Method of Diagnosis** - Close the clubface until the ball launches straight.
 * 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.
 * On an iron, an extreme push, especially if it's inconsistent or won't straighten out by closing the face, indicates a slightly "shanked" shot (the ball glanced off the hosel before hitting the clubface).

**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 would normally cause a straight slice), coupled with either a slightly closed face to compensate (somewhere between square to the intended line and square to the swing path) or a hit off the heel of the driver. 

**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 wrists on the backswing; that will typically cause you to have your hands too far forward at impact which will open the clubface (and also makes the shots much less consistent, as you have less control over the clubhead on the downswing). 
 * 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 mishits:**
 * If you're hitting off the toe, get an inch closer to the ball as you take your stance. If you're shanking, get an inch *farther* from the ball as you take your stance. See the below section on making sure you address the ball properly.
 * 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 of your club adjusted (if possible).

* **To solve an open clubface at impact:** 
 * 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. 
 * 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.

* **To solve swing path problems:** 
 * First, make sure you address the ball properly: 
      * The ball should be placed inline with the inside of the arch of the left foot for the driver, an inch or two back for fairway woods, and around your center line for most irons. 
      * Stand with your feet parallel or slightly splayed, a little more than shoulder width apart (it should be comfortable to stand this way and should not restrict twisting at the hips or waist). 
      * Bend at the hips until you're looking at your toes and your weight is evenly distributed along your feet between toe and heel. Bend your knees slightly; don't squat, don't lock them. Let your hands hang naturally. 
      * Ground the club in front of you, with the shaft along your centerline, the face square downrange, and your hands still hanging naturally; the shaft should be pointing at your belt buckle. This will be the lowest point of the swing arc.
      * The ball should be just a smidge inside the centerline of the club and should be teed up to place the top one-third to one-half of the ball above the level of the striking face of the grounded club. Both of these adjustments are to compensate for the ball being forward of the bottom of the swing arc.
 * Then, make sure you're swinging the club though one plane of motion:
      * 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. Practice at the range to straighten the arc and dial in the impact zone again; with the lower backswing and your normal swing arc, you'll have a tendency to "thin" or "skull" 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 dropping the arc 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 bring your hands in to make contact. Again, practice at the range to avoid "turfing" the shot with the higher backswing and your normal arc.

* As a short-term solution to most of this, rotate the club counterclockwise to close the face until the shot flies straight, and then correct your aim left or right to compensate for the swing path. While this will produce a straight flight, the misalignment can still cause distance problems, inconsistencies, etc and so you should spend some time at the range adjusting your mechanics.