Players have always had different service motions. If you put 10 players on the baseline at once and watch them hit a serve, you will see 10 different serves. There will be similarities and there will also be many things that the best servers have in common.
The toss will be consistent, the shoulder turn will be at least 90 degrees or more to the net, and they will land inside the baseline after making contact with the ball.
The biggest change that most of the best servers in the modern game have made is that their backswing is shorter than the great servers from 20 years ago. My advice is to try this since it simplifies the motion and tends to be easier to repeat.
Start with your shoulders sideways to the net (or slightly past sideways) with the racquet pointing towards your target and the ball touching the racquet. Turn slightly and let the racquet drop just a couple of inches before bringing your hand up towards your serving shoulder. Try to toss the ball straight up in the air as the racquet is moving towards your shoulder, keeping the racquet moving smoothly all the way to the contact point.
Remember the keys I described above that the best servers have in common and you will reap the benefits of this simplified motion and serve.
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