Aside from reducing RPM and increasing engine load (easily the best way to improve this), I would think another way would be to just reduce the volume on the other side of the throttle plate(s). One could:
1) Reduce plenum volume
2) Reduce runner length/volume
3) Advance the intake cam considerably, so the valves close early, meaning more of the vacuum production would be inside the cylinder and less in the intake manifold (smaller volume)
4) Individual throttle bodies (ITBs)
