Look at your trophy! Usually it is a figure of a server. Look at the knees. They look like they are bending toward the net. Look at the back of the figure. It looks like it is "coiling" away from the receiver. Look at the toes of the figure. The heels are off the ground and the toes are carrying the weight. By doing these things you will prevent lower back pain, you will get more vertical on your serve (because you are forcing yourself to use your legs), and you will start "coiling" your body like a pro.
Lastly, look how the toss arm on the trophy goes to full extension, beautiful.
Remember, after you throw your toss -- coil and bend your knees simultaneously, lift your heels, leave your toss hand up as long as possible, then climb to hit the ball.
Look like the trophy when you hit your serve.
Good luck on the courts!