Drill #1 Warm Up & Coordination Drill
Part 1) Just getting a few nice and easy overarm / baseball type of throws to a wall or partner to warm up and loosen up the shoulder. Focus on the kinetic chain.
part 2) Toss and throw. This works on coordinating your toss and eventual serve. Note: when you throw, do it with the intent of throwing it far, eg, across the net or for distance. Don't cheat and just try to hit the ball for the sake of hitting the ball.
Drill #2 Toss & Trap Drill
Part 1) Identify contact point. This can be done against a fence on court. Reach up high and gently place racquet on fence/wall
part 2) Toss the ball slightly above the contact point so that you can watch it come down to the contact point and trap it gently with your racquet. This drill will help you be more aware about your tossing and contact point. NOTE: Keep the toss really close to the fence or wall, or else, it will rebound and it'll be hard to trap it.
Drill #3 High 5 Tap Drill
Part 1) Get used to stopping the swinging arm at contact. Swing arm as per serve motion and try to catch the ball at contact point. NOTE: If you can't catch the ball, that's completely fine. You can just high 5 it with your palm but make sure there's no follow through
part 2) Repeat motion but with a racquet this time. This drill is really really useful in helping you identify your contact point and work on your reach. The ball should be able to travel forward and not just smacking it down.
Drill #4 Knee Bend Drill
This drill works on the coordination of parts of the kinetic chain. We generate power from the ground up. Everyone has their own optimal knee bend angle, so a good way is to practice is to test out different amounts of knee bend. It should feel like a rubber band ready to fly, if you feel like it's sapping energy instead, then maybe you have a little too much knee bend.
Drill #5 Infinity Serve Drill
This one takes a bit of practice. It helps to warm up your shoulder and practice your object manipulation and fluidity of your serve motion.
Step 1) Use your racquet to used to the swing path and rhythm.
Step 2) Use a soccer sock, place 1-3 balls right in it. The goal is to make sure the sock doesn't have any slack during the swing path. It should be extended through out. This will seriously give your shoulder a good work out and train your body to learn how to feel and manipulate the weight of an object. Once you get the hang of it, you can practice to fling it for distance.
NOTE: You may get hit by the sock, but it's all a lesson in object manipulation
The throw in the end is to give you a focus, i.e the end of the kinetic chain, which is the release. Remember, power comes from fluidity and not how "hard" you try to force it.