An excellent way to naturally move the racquet in a low to high direction on your forward groundstroke swing begins with the racquet head coming back high.
Although this sounds like it goes against the low to high idea, the high racquet head helps create a "loop" which is what many pros consider to be the key to generating topspin. The racquet has to drop in order to make solid contact and a natural low to high swing is the result.
Try starting in the ready position with the racquet head high, use your shoulder turn to get the racquet back (keeping the racquet head high), and then move the racquet to the ball for a solid hit and a simple swing with a great result.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis (http://www.annaconetennis.com) and MyHamptonsPro (http://www.myhamptonspro.com). For details on lessons and clinics throughout the Hamptons contact firstname.lastname@example.org or email@example.com or call 865-300-7323