I had a horrible slice for years.  I always just played it off the tee because trying to fix it was killing my score.  One day I played a round with the club pro and asked him to watch my swing.  He had me tee up about 3 balls and watched.  Low and behold, I had an outside - in swing.  When I asked him how to solve it, he simply said, "slow down and keep your right elbow tucked".  I am right handed by the way.  As it turns out, it fixed my slice.  I still slice every now...