Light is of prime importance. While it is true of all photographs, it is much more so of night pictures. But you can get creative in night pictures with light. Don't always try to capture what your eye sees, but aim for an effect - the effect may something you wouldn't normally do in the daytime. Also, if you have a remote flash or any light source that you can move, there are countless ways you can use it to change your composition (if your subject is relatively near you). You can create some amazing shadows too.
Light from a single source works with an inverse square relationship - so if object A is twice as far from the light source as object B, it will receive 1/4 the light. What this means is you can have very strong gradients, and can easily put the visual emphasis on what you desire. So get creative with how you see and use any light already there or that you bring.
Here's an example of a night shot I took today that breaks many of the rules (even has a lot of noise in it intentionally):
