From my experience:
Keep the aperture as large as possible. Keep the ISO down (<400) ! Buy a remote and experiment with the timing. You'll probably want a few mins if there's no light directly involved (i.e.: streetlamps / car headlights).
When you get the pic on your comp, you'll probably want to cool the temp down as the above are all a little orange.
Light trails are fun..
Random shot I took in Plymouth one time:
Doesn't have to be pitch black to take a funky shot either:
I attempted star trails when I was in Cornwall once.. 15 min exposures are REALLY boring to wait while they are taking haha.