Copying Pete's serve is always a good idea.
Except his little foot raise before his motion can lead to some annoyances.
The key to really having a good serve is body rotation and flexibility. You can only unleash your full power when you swing your hips and body in perfect order. Full hip rotation is the key. In addition to the points the others here made... swing your shoulders with your body, loosen your grip, and pronate at impact.
Knee bend is fine... but it's not something you should be focused on. If you overdo it, it can affect your balance and timing. Unless you're a total stiff, most people naturally bend their knees for their serve, and that's fine. Don't copy Becker's knee bend. Never copy anything about his serve.
Also, I DON'T suggest doing the "A Faster Serve with Rick whatever" drill, especially if you're a beginner. He makes a good point on rolling your shoulders, but that can further grow some bad habits.
The one thing I always see with the people I work with:
They don't fully reach up for the ball.
In the Sampras tape, "Your contact point should be out in front of your body, and you should be able to draw a straight line from the top of your racquet, down your arm and to your front foot."
It takes tons of practice and good coordination to learn how to do this properly. Lots of people can't. Even some pros like Schalken can't.