I watched a video recently by a coach in the UK. He was very pedantic, in the old-fashioned way. One great tip that I have started incorporating is about body rotation. He says the way to make it automatic is this: in the deuce court for a rightie server, stand with your left foot angled at 45 degrees or towards the net post, as usual, but face the target, i.e. the deuce court on the other side. Then the body coil becomes necessary!
After a while, you will coil your body even in the ad court by habit!
Coiling achieved with no effort!
The other great insight he provided was that rocking motion (weight on front, back and front feet) automatically causes knee bend!
These are two of the best serving tips I have ever found.
His third tip was to open up the racket face as it passes behind the back foot. This will cause automatic shoulder rotation. I agree, but I find that it might result in more spin but less pace.