I warm up and practice my serve this way. Nice loose arm at first, not much leg. Get the arm loose, then get more shoulder happening, then get some leg bending. Not hitting hard - just trying to feel the racquet whip a bit. Depending on how I feel I might do this for a few serves to a few minutes. Eventually I start getting the form more correct while keeping the nice loose arm and adding power.
I find that if I start with hyper-correct form I can be very tight in my shoulders and arm - takes away from the power.