I'm trying this right now (in post)... I seached the vegas forum at creativecow.net (make sure you search archived posts from previous years) and found some threads on this - both on how to shoot, and how to do it in post.
I've experimented a bit by using the HSL filter to reduce the saturation of a clip, then overlaying a blue gradient. The results, of course, depend greatly on the original clip.
Use the Tiffen "Day For Night" filter. Just make sure to avoid dead give aways like no light pooling in front of headlights. You will also have to do the tweaking the other folks mentioned. I always strive to achieve effects with the lens, rather than spend hours in post. Our turn around times are so short that even with our AVID Media Composer we can't generate all effects electronically. You might also try combining a Tiffen Soft FX 1 or 1/2 with the Day for Night filter. THe Balck and Pro Mist filters will give you an undesirable blooming effect whereas the Soft FX just smooths the image (also good for masking the signs of aging).