Important point not yet mentioned (unless I missed it) is that to make the ball short, you have to impact the very bottom of the ball. Impact the bottom of the ball and make the first bounce near or past 1/2 way to the net, you will get a short serve.
I have to say that while heavy spin is nice, it is not what wins even close to 1/4 of your points. Spin variation not detected is the key.
How to improve the spin???
This is a function of accelleration and speed at impact with effective timing using the whole body to help control and nice use of wrist. You do not need a huge backswing.
Several ways to improve your timing and ability to contact bottom of the ball.
If you are at the table...
1) Stand up, toss ball, impact higher than net level on purpose and do a slow to medium short swing with wrist to impact the bottom of the ball and make it come back. Do not worry if it goes over net or not, you are practicing the impact and timing, you have to get accustomed to striking bottom of the ball and to get spin before you can worry about how to place the first bounce. Keep practicing this for days until you get the hang of hitting the BOTTOM of the ball and getting the timing for a short, slow to medium speed swing. You are not trying to run 100% here, just get the feel of the strike zone and how to time/impact in an easy situation without pressure.
2) Serve and impact bottom of ball with medium speed. Hit ball in air above net and land 1/2 way. You will get short serve, even if the ball is way too high. This is not the time to worry about hight as the serve isn't yet match ready, you have to take it one day, one week, one month at a time. Depth control, height and placement are not yet the objectives, don't even try for perfection yet, it will only get you upset as you will fail. Focus on impact, timing, spin, and simply keeping it short. You can train the rest well enough later. You have to take it one step at a time.
3) Once you can consistantly strike the bottom of the ball, you can now experiment with how high in flight you strike the ball and where you make the first bounce. Focus on how the result changes as you change impact height and first bounce location. You already now have dependable impact and timing for a slow to medium racket speed. Keep racket speed at medium and try not to go for too much as the timing is too dificult at this stage. Focus on learning and discovering your impact zone and how to keep the height reasonable and the double bounce where you want it. The maximum spin is not yet the objective, it comes later.
4) Try a swing with more acelleration and speed, but sacrifice you height control. Once you learn how to make good bottom impact with a faster racket speed, you can discover how you make impact hight, first bounce, and body contro, after movement into ready position for 3rd ball.
5) Once you got all this down OK, you can test how much spin you generate by placing a towel or something soft with friction where your 2nd bounce lands. You will see if your spin is heavy or not. You can use this as feedback for how well you vary your spin, whether it is light, heavy, or "Heavy" no-spin.
6) You have been using the same service motion all along and now is a good time to try learning another serve motion or two for a variation.
IF YOU ARE NOT AT THE TABLE... You can still practice impact.
1) sit down, toss ball up, impact bottom with arm & wrist, let ball go out less than 1 meter, make ball spin back.
2) Stand up over floor, toss ball, make your chop serve, and make ball come back. if you are not getting enough spin, you are not impacting the bottom or your timing and racket speed at impact are not yet there.
Don't feel bad if it doesn't all come together as fast as you want it. We are all competitors with a spirit, but reality is always different from our expectations. The spirit keeps you wanting it and any of us can get there.
These forums and modern vid sharing sites are great ways to learn and discover new ideas.
SEE YOU ALL AT THE TT CENTERs doing Ur thing.
William.