Tips for Long-Exposure Photography:
1. Use a sturdy tripod, one rated for the weight of your camera and lens (over-rated is better).
2. If your camera has a mirror lock-up feature (many lower-end, and even mid-range bodies don't), frame your shot, then lock-up your mirror prior to tripping your shutter.
3. Buy the wireless remote release (typically, not that expensive) made for your Nikon body, and use it to trip your shutter remotely.
4. If you don't have the wireless remote, you can also use your self-timer to trip your shutter.
5. Weight the center post of your tripod with a shotbag or sandbag for increased stability. While video tripods sometimes have a hook for this purpose, you'll probably have to jerry-rig something for your still tripod.
6. Shield your camera from wind using an 18" x 24" solid or piece of Foamcore (or whatever's handy), to eliminate shake if shooting under windy conditions.
7. If using a VR lens on a tripod, turn VR off. If using a VR lens on a monopod, turn VR on.
No tripod or monopod was handy for the image below, so I rested my camera on a handrail, and steadied the shot as best I could:
AF Nikkor 85mm f/1.8; ISO 640; f/1.8 @ 1/25th second.
For the photo below, again, no tripod or monopod, so I cranked the ISO to make up for it (not recommended).
AF Nikkor 35mm f/2.0D; ISO 2000; f/2.0 @ 1/200th second.
Finally, this time I brought a tripod--my tripod legs were splayed wildly over this stairwell at LAX to get the angle shown below. And, because I had a tripod, I was able to shoot at ISO 100, and stop-down all the way to f/11 for this eight-second exposure:
Tokina 11-16mm f/2.8 DX; ISO 100; f/11 @ 8.0 seconds.