One important aspect to remember while executing any and all serves is to stay fluid and relaxed throughout your motion until you add power simultaneously to your upper body thrust and your arm pronation as you approach the contact point.
There are several major types of serves: The Flat, Spin, Topspin, Slice, American Twist, and Andy Roddick's "New Millennium." He takes his racquet straight up and back until his hand is somewhat behind his head. Then he whips forward and produces tremendously powerful serves.
Many amateurs unknowingly use an underspin serve which is never seen on the Pro Tour!
But we must also include what I refer to as the Baseball Serve. This serve affords a variation that is able to be used with most of the serves previously mentioned other than the American Twist. Therefore one almost doubles the variety of bounces your serves make.
Vic Braden made me (in my early 60's) aware of the Baseball Serve when I viewed him on tape demonstrating how easy hitting an overhand serve could be if one only served just as though they were throwing a ball. This means using a lower contact point. I used this method that same evening and my "tough to return serve" immediately won many 40-love and 40-15 games.
Many of us started throwing things when we were little kids and we have thrown untold thousands of balls and other objects! Your lifelong throwing motion means you don't extend your arm for a high contact point, but just rotate your body and pronate as you snap your arm and wrist to swing your racquet head through the ball.
To initiate your use of this serve:
Try throwing several balls being very conscious of all aspects of each and every one of your body movements so you will able to repeat them exactly. Now, pantomime using the very same motion you have just used noting that your hand passes your head only a little above your ear! Then measure while holding your hand where it passes your head when ball tossing with your racquet face up where the lower toss ("Put") position should be for this new "throwing" serve. Now make a few easy attempts to execute your new serve!!
I hope everybody is as pleasantly surprised as I was at how much more power I could immediately produce by using my many years experience of throwing softballs, baseballs, and stones.
No baseball player nor any experienced pitcher would ever be able to produce as much power by swinging their totally straightened out throwing arm while making a pitch.
Those amateurs who are able to adapt and use this serve will immediately have doubled the total number and type of the serves with which they previously had gained proficiency.
I have seen Lindsey Davenport use both the high and low contact points when serving in matches. Maybe that is because Vic Braden has conducted a Tennis Academy in Utah for many years and excellent clinics all over the west coast area.