The Correct Grip Position
See also How to Find Correct Grip for You, How to Grip with Your Left Hand, and How to Grip it with Your Right Hand.
If you are a slicer, you might have a weak grip. Lots of golfers start to hit a draw after they changed their grip to stronger grip.
But some golfers still slice the ball even after they switched to a strong grip.
Inside of your left Thigh
The reason why they still slice the ball might be the position of their grip (hands) at address.
Slicers tend to position their hand in front of a zipper at address. This will cause the clubface to open at impact.
Why?
Well, at impact, your hands will be little more ahead than that to add power to the ball.
Try to push your golf bag sitting on the ground with your 7-iron for an experiment. Try to push the golf bag forward using the clubface. When you do so, look down to see the position of your hands.
I think that nobody's hands are in front of the zipper because you can't add power in that position. Your hand must be little more ahead than that.
Try this also. Set up to the ball with your hands in front of the zipper. Without moving your clubhead, try to move your hands forward.
What happen?
Your clubface opens when you move your hands forward.
So to put it in a nutshell, you will slice the ball if you position your hands in front of a zipper (or in the middle of the stance).
To hit it straight or a little draw, position your hand toward the inside of your left thigh. If you are not getting any results after changing your grip, check the position of your hand at address.