If you watch professional tennis, it seems that there are a lot of different ways to follow through. Although players tend to have different finishing spots and techniques, most have a lot of things in common on their follow through.
First, and most important, the pro players tend to let the racquet continue through the point of contact and go as far as it naturally feels comfortable. Keep in mind that the racquet position is not as important as where the players' hands end up. The finishing point for the hands should be close to the opposite shoulder. Finishing way below the shoulder is rare and finishing way above the shoulder is also unusual.
Club players will look at Rafael Nadal's exaggerated high-reverse follow through (over the same shoulder as the racquet started from) and try to copy that. I would highly discourage this since it takes incredible timing and strength to execute the shot as Nadal does. Instead, try letting the racquet go to its natural position near the shoulder and repeat the same idea over and over.
The consistency of the follow through will improve the consistency of the shot and likely produce more power, depth and accuracy.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis (http://www.annaconetennis.com) and MyHamptonsPro based in Sag Harbor, NY (http://www. myhamptonspro.com) and Tennis Professional at Ventana Canyon Golf and Racquet Club in Tucson, AZ. For details on lessons, clinics, or coaching, contact Steve at firstname.lastname@example.org or call 865-300-7323