These images have been posted many times here, but perhaps they will clear up misconceptions about Pete's serve.
In this sequence you can see that his contact point was well into the court:
While you have to look closely for the ball at contact in pic 7, it is about 2 feet inside the baseline.
Note also how Pete really throws his left side into the court in a cartwheel movement, while you don't.
That is at contact in pic 7, his right shoulder is pointed straight up and his left shoulder is pointed straight down.
Here is another view at contact to emphasize how vertical the shoulder orientation is at contact:
And yet less than a second before in his trophy pose, it was his tossing shoulder that was straight up while his hitting shoulder was straight down.
So that rapid reversal of the shoulder angle, or cartwheel, propels the whole left side of the body almost straight up in pics 1-7 above, but also into the court as well.
Because he throws the left side of the body so forcefully into the court, he lands on his left leg, with his right leg kicking straight back in pic 12.
Will Hamilton of Fuzzy Yellow Balls tells you how to land like this - when you learn to really hit "up the mountain".
Leg Kick on Tennis Serve http://blip.tv/fuzzy-yellow-balls/le...-serve-1190196
"Up the Mountain" excerpt form Serve Doctor presents: M.P.H. http://www.youtube.com/watch?v=WlPVdppfYGs
I think the above will be easier for you to work on than the following which usually presents more of a timing problem.
But in addition to the source of power that Sampras gets from that big cartwheel movement, he also has a huge coil/uncoil movement.
From the pics above you can see that while tossing, and shortly after he releases the ball he literally turns his back to face court he is serving into.
That is how he is leading with his back in his trophy position in pic 1 of the first photo sequence at the top of this post.
Finally, on that first sequence note that Pete gets WAY up there from his powerful leg push off. He has some serious hang time in pics 7-10 above.
As others have pointed out there is only one Pete Sampras. It is great to add elements of his serve - bigger leg push off, bigger cartwheel, and bigger coil/uncoil to your serve. Just don't expect as a mere mortal that you will exactly duplicate his motion.
Still, you might also get a kick out of Ian from Essential tennis in a comparison of a pretty impressive duplication of Pete's serve compared to the original. I think you might find some of Ian's commentary would provide additional hints on how to improve your serve.
Sampras Tennis Serve Comparison http://www.youtube.com/watch?v=jDHDS9G9sKw