  We discuss a geometrical method to define a preferred reference frame for
precessing binary systems and the gravitational waves they emit. This
minimal-rotation frame is aligned with the angular-momentum axis and fixes the
rotation about that axis up to a constant angle, resulting in an essentially
invariant frame. Gravitational waveforms decomposed in this frame are similarly
invariant under rotations of the inertial frame and exhibit relatively smoothly
varying phase. By contrast, earlier prescriptions for radiation-aligned frames
induce extraneous features in the gravitational-wave phase which depend on the
orientation of the inertial frame, leading to fluctuations in the frequency
that may compound to many gravitational-wave cycles. We explore a simplified
description of post-Newtonian approximations for precessing systems using the
minimal-rotation frame, and describe the construction of analytical/numerical
hybrid waveforms for such systems.
