The biggest culprit behind a slice is an open clubface at impact. Sure, the path of the club plays a role, especially in determining the severity of the slice. But to banish the banana ball for good, you must fix the face first. Here's a simple way to do that before you even take the club back.
1: POINT THE FACE
Take your normal stance, and then using your right hand only, sole the clubhead on the ground opposite your trailing foot. Make the clubface point down the target line, with the grip pointing at your belt buckle.
To lock the clubface in on the target, sole it behind the ball with your right hand only.
2: TURN THE TOE IN
Now bring the club up in front of you— again, keeping the shaft in line with your belt buckle—and add your left hand to the grip. Make sure your palms face one another and that the "V"s formed by your thumbs and forefingers point toward your trailing shoulder. Now look down at the clubhead: The face should appear slightly closed, with the toe turned in.
With the clubface in position, lift the clubhead off the ground and add your left hand to the grip. The clubface should appear slightly shut.
3: PREPARE TO HIT IT STRAIGHT
Set the clubhead down behind the ball without changing the position of your hands on the club. Taking your grip this way—with the clubhead starting by your trailing foot— presets the clubface in a slightly shut position, offsetting your tendency to open the face at impact. It also promotes a stronger left-hand grip and more forearm rotation through the hitting area, which can also prevent the face from opening.
Now simply set the club back down behind the ball. The slightly closed position of the clubface should help offset any tendency you have to open it at impact.