You slice or sky the ball because you slide or tilt and turn your body towards the target too early and excessively during your forward swing. This can cause too steep of a downswing, especially so when you are using your driver.
You need more power and push or slice the ball because you have too steep of an out-to-in swinging path.
You need to find a way to try to cure your slice and learn how to feel the release of the club. Your slice could be caused by gripping the club too tightly or using a swinging pattern that is prohibiting you from squaring the clubhead at impact.
Slicing the ball because your swing is too upright and from the outside in.
You slice the ball more times than you hit it straight. So how should you set up to the ball when you want it to fly as straight down the fairway as possible?
Hitting off of the heel of the club with your woods. Pushing or slicing your irons due to a blocking action with your irons.
You constantly hit weak fades and slices. A likely cause is that you pick the clubhead up during your backswing, causing you to cup your wrist on your trailing arm. Then your lead hand is unable to square the clubface before making contact with the ball.
You are slicing, topping the ball or hitting it thin because you teed the ball too low towards the ground.
You slice severely because you cut across the ball on your tee shot. Your might have lifted your forward heel up off of the ground during your backswing or allowed your back shoulder to lean forward during your forward swing, causing you to swing over the top.
You lose distance on your drives due to a severe slice. This might be from an exaggerated open stance or an out-to-in downswing with the clubface open. Another possibility that is often over looked is the driver that you are using. It could have too stiff of a shaft, be too long, or it may not have enough loft.