Brake the hip, was cruicial
As I couldn't move, the only thing I could practice was serve... 100s and 100s times... Body twist or forward motion was also a no-no, so I focused on tosup, racket motion and accelleration. Pretty decent topspin serve evolved.
As the hip got better (and I grew bored of the slow bouncy seeves), I started tossing that ball forward and really putting the hip/boddy forward, for strong "bow" effect. Flattened the serve too.
In the late summer it somehow fell appart again, especially switching between fast 1st and topspin 2nd. Then it came back with a vengence again, especially the fast 1st.
Somwhere in the process I learned to relax the wrist and alow the frame to snap forward.
Latest development is experimenting with gut/poly hybrid in heavier frames. This now requires body coilin/uncoiling too, in order to not pull my shoulder out of the socket. First I sprayed all over the place, but it came together again, now with evil side spin too, when wanted, but maybe a tad slower...
I still prefer my medium weight racquets for serving, though.