for me the swingpath is much like a saucer starting high sloping down staying more level than down thru contact then sloping back up. this has helped get a more driving slice and not a chop or floater. if you watch a pro slice and stop frame it you will see that even tho on take back the racquet is angled up as they come into the ball the racquet gets more parrallel to the court
Every tennis pro, instruction book and DVD I have consulted would agree with this. Yet, there are several posts in this thread recommending a strict high-to-low path for the racket, rather than a smooth high-to-low-to-high path. High to low just produces a chop, not a slice.
The other mistake to avoid is exaggerating the change in height of the racket. The entire high-to-low-to-high path does not change much in height from the highest spot to the lowest spot. A good slice is driven forward, not with excessive racket speed, but not chopped, and the swing is fairly level.