Jim Stone talks about how to adjust your platform on serve receive based on the trajectory and speed of the serve.
First, keep the arms relaxed, and don't prematurely form your platform until your lower body has adjusted to the serve location. This will allow for last-second adjustments.
Second, keep the posture relaxed. A passer should never be stiff, as they constantly have to readjust to the serve before actually making contact. Instilling a calm approach to serve receive in your players at an early age will set them up for future success.