try to shoot at 'magic hour' no direct sunlight casting hard shadows, try to avoid any light emmitting sources (car head lights, street lamps etc) unless you dont mind the extra work in rotoscoping. Also becareful of strong reflective surfaces in the shot as they can capture the sunlight in a way which can give you problems when attempting a convincing day for night. when it comes to the post production, check out
http://www.videocopilot.net/tutorials.html?id=1
Hope this helps