LeeD gave you the answer, but it seems a little more explanation is in order to understand "Followthru in the direction you want the ball to go."
When we serve, we throw the side
of our body at the court we are serving into.
So in our trophy position (pic 4), we are in a bow shape
with our front hip over the baseline, and our upper body tilted backwards at the back fence.
Our tossing shoulder is straight up, and our hitting shoulder is straight down.
As we serve we throw the right side
of our body up and forward (pics 5-eight) to impact.
Our shoulders should "cartwheel" vertically
so that by impact (pic eight) our hitting shoulder is now straight up, and our tossing shoulder is now straight down.
The "bow shape" is now reversed - with the upper body now in front of the hips.
You can really see this cartwheel
shoulder over shoulder action and the bow shape
/reversing the bow shape in the following video of Soderling serving:
Robin Söderling serve slowmotion http://www.youtube.com/watch?v=a56pvP1i6x8
[You really need to have this vertical
shoulder over shoulder motion, both to increase your serve speed and provide direction to your serve, and to avoid a rotator cuff shoulder injury:
Preventing Rotator Cuff Injury http://www.youtube.com/watch?v=lTRvxaBMh8s
Now to serve to the right "cartwheel" your shoulders slightly more to the right.
To serve to the left "cartwheel" your shoulders slightly more to the left.
Ian from essential tennis shows in this video that it takes only a subtle difference in the direction you cartwheel to get your serve to go either wide or down the middle:
Pronating on Wide and Middle serves:http://www.essentialtennis.com/video...e-serves/1723/
[Don't be dismayed on his talking about pronation -we all know we need to pronate - but pay attention to the slight difference in the wide down the middle serve.]
I hope this helps.