GetBetterer was good enough to post a reference to my advice on serve direction, but I thought it might be helpful to have you consider the following, if you are one that doesn't click on every link:
"The key in serve placement is how you "throw your upper body" in the direction you want to go.
Watch the following video by Ian Westermann of essentialtennis.com:
http://www.essentialtennis.com/video...middle-serves/
The main point he makes by using 2 cameras from the back and side is that for his toss and trophy position there is very little difference in body or ball toss for a wide, body, or down the T serve.
The difference comes from the way he swings his upper body forward out of the trophy position.
Since his arm is attatched to his upper body, this means he doesn't have to change the arm is swinging, only the direction the upper body is moving.
(It is likely you already know this, but in your trophy position you assume the shape of a bow with your front hip out and your upper body and hitting shoulder are directed back, and your hitting shoulder is much lower than your front shoulder. As you swing forward, your upper body "cartwheels" forward with your hitting shoulder ending up much higher than your front shoulder. The direction you "cartwheel" is the direction your serve will go.)
If you are a disciple of the "serve up the mountain" concept
http://www.youtube.com/user/servedoc#p/u/5/WlPVdppfYGs just move your mountain a little to the left if you are a right hander trying to serve out wide on the deuce court, and move your mountain a little to the left to serve down the T. Then just serve up your particular mountain for great placement of a powerful serve."
In many ways, this is a rephrasing of gzhcpu's post above "I remember Pat Rafter saying he would just fall into the court in the direction he wanted to place the ball..."
But Pat Rafter, just like Soderling in the following video, really threw himself into that fall with a big cartwheel:
http://www.youtube.com/watch?v=a56pvP1i6x8