It's often difficult to judge how much spin you are putting on the ball when you practice serves. Without this feedback, it's not easy to improve your serves. So try this - find a large, carpeted room, and practice serving there. Spin the ball as if you were serving and try to control what it does on the rug. With backspin, make it roll back to you. With sidespin, make it curve sideways, perhaps aiming at targets. (Perhaps have a little fun by having it curve around objects.) Keep working on getting more and more spin, while still controlling it. To maximize the spin, accelerate the racket through the ball with the arm and wrist, and just skim the ball. When you can get huge amounts of spin this way, and can control it, then try it on a table.