PCB Layout - Ground Plane Return Path
This sounds odd -- but try not partitioning your ground plane.  Instead, make sure that the components and traces for a given subsystem (logic, analog, digital) all stay in their respective areas instead of "spilling over" with high-speed digital traces running through analog circuits for instance.  Ground plane currents will take the lowest impedance path back to the source, which at high frequencies follows the trace they are the return for.