There's only one thing that can cause a slice, and that's a clubface that's either open (or opening) at the point of contact. That being said, here are three tips to help you square up the clubface and rid your game of that slice forever!
Get A Stronger Grip. The clubface tends to return to the ball open when the hands are placed on the club in a weak position–that is, turned too far to the left. A correct grip has the hands rotated more to the right. To strengthen your grip, rotate your left hand so that your thumb is positioned to the right of center (two to three knuckles should be visible). The right hand also should be rotated to the right, matching the angle of the left hand. To experiment, turn your hands to the right until the ball begins to hook, then back off a little for optimal positioning.
Ease Up! Excessive pressure in your hands and arms inhibits the natural rotation of the clubface through the hitting area. Soften the pressure in your hands, wrists, arms and shoulders to encourage a more natural, effortless face rotation. If you imagined a scale running from zero to 10, where 10 was the tightest you could possibly squeeze the club and zero was the club slipping out of your hands, then the ideal pressure for most shots would be a 3.
Flatten Your Plane. Most slicers approach the ball on too vertical a plane, another error that facilitates an open clubface at the point of contact. A flatter swing shape will promote a natural squaring of the clubface and create the preferred right-to-left ballflight. To sense the feeling of swinging on a flatter plane, make some practice swings with the clubhead moving back and through at knee-high level. Swinging the club in this elevated position will help you feel the more rounded swing shape needed to allow the toe of your clubhead to rotate past the heel. After a few of these baseball swings, try one off the ground with the same feel. Your ensuing ballflight should be much straighter and, perhaps, curve slightly to the left.