We haven't worked that hard on it specifically though we usually have the radar on which I think provides good feedback if not overly fixated on (we try to work spots and spin as much as speed). Early on, if I'm honest growth was the biggest contributor though we did work on loading the push leg and exploding out. After that she got a feel for IR and she generates good whip fairly naturally. In the last year, she gained 3-4mph and I think nearly all of that was getting a feel for front side resistance and especially what her coach calls shift where the glutes activate, there is abduction (or adduction: somebody help) and the kinetic chain transfers energy from the bottom half to upper. Was hard to learn but that was huge on speed when she started to get it. She naturally has a fast/strong arm which helps though she's not very tall. The other thing I think we have done well is take time off and avoid energy. Avoiding a 6 month break to shake something has helped her be able to consistently improve.
It's a huge threshold to cross for a pitcher. It's amazing how many dads tell you their daughter throws 60, but when you see them throw or ask for proof, you find a lot are exaggerating.