How can I reduce the graininess and noise resulting from high-ISO settings and slow exposures in night photography?
Well, the easiest way would be to use a lower ISO, but you could check if your camera has a noise reduction setting that you could turn up.There are more tips here http://www.anandtech.com/digitalcameras/showdoc.aspx?i=2351&p=2
My D80 has noise reduction feature. For both High ISO settings (ISO 1600) and long exposure. I swear that camera is awesome!
if using film overexpose and reduce developing time. That should help, as well as deal with the tendency of long exposures to have high contrast