err.. I think you should wait until the moon is high in the sky I guess, so there is less atmosphere to travel through. A clear night helps, not just without clouds, some nights are much clearer than others. look for the stars to see how clear the night sky is.
Other than that, for me is just get a 300mm lens, ISO 200, f8, 1/640. Although I used the same model of lens in those two shots, the quality is quite different. could be the night, could be the lens, could be the camera.