I observed players of different skills and I noticed that one repeating mistake is in the fact that it is hard to throw the ball straight up, I have the same mistake. When serving with the ball going straight up, serving becomes far more consistent (not hitting the net and not going over). I simplified the serving to the serve like in Pesäpallo (i.e. you go down in your feet while your hands are at the constant height, not to get the release of the ball in circular path so only trying to get vertical velocity). Now my couch said it does not look like tennis even though the shots became far more consistent. So:
How to throw the ball straight up in tennis's serving?