Personally I think night photography is the most difficult to master (and by no means have I mastered anything about it!). But here's a few tricks to help get your long exposure shots to a higher standard.
Aperture - To obtain a sharp photo, a smaller aperture is required (which is a larger number on your camera - F/8 is a smaller aperture than F/5.6). However, during night photography this is problematic. The smaller the aperture, the less light your lens allows into the sensor, thus a higher ISO (sensitivity to light on the sensor) is required. So a setting of ISO 1600 might be enough to have an F/8 aperture for 4 seconds, but if it's dark, moving the aperture to a larger setting, like F/5.6 or F/3.2 would be beneficial. I’ve found it helpful if some natural ambient light remains to fill in colors and details. Aperture is probably the least important setting for night landscapes, but if set improperly, can affect all the other settings!
Shutter Speed - One requirement for night photography is a tripod since the shutter is open for longer periods. The rule of thumb for sharp photos is the shutter speed must be greater than your lens focal length. For example, if you have a 50mm lens, your shutter speed should be at least 1/50th of a second. I usually double that amount since faster shutter speeds almost always give you better photos. Of course having a tripod will greatly increase your chances of capturing a very sharp photo, and is required for all night photography.
Back to shutter speed…if your camera is set on aperture priority, the shutter speed doesn’t matter…the camera will choose it. But in night photography, YOU want to control the amount of time your sensor absorbs light for the proper exposure, and it’s necessary to govern how much light reaches your sensor by using shutter speed as the variable. To begin, set your camera on shutter priority for 5 or 10 seconds, ISO 1600 and the camera will choose the aperture. If the scene has some amount of ambient light, these settings might be enough to bring out good details and color despite the darkness. Change your shutter speed accordingly depending on how under or over exposed the photo is.
ISO - This is a tricky setting since each camera has its own limits on how high you can set this. In general, the higher the ISO setting, the more sensitive the sensor is, however, the tradeoff is that “noise” is introduced as a by-product (dots, discoloration, grain, etc). Full frame sensors have the least noise in low light situations even with higher ISO settings. For example, using ISO 3200 on a full frame camera, noise will be slightly visible, but on a crop sensor camera (the most common for lower-range cameras), that same setting would create quite a bit of grain/noise. For those of you who really want a clean look regardless of what kind of camera you have, try purchasing some noise reduction software to use in post-processing (I use Imagenomic as a plug-in to Photoshop). It’s amazing what a difference noise reduction software can make.
Tying it all together - In the above photo, it was a dark night except for the Northern Lights, so a higher ISO was required to keep a 15-20 second range for shutter speed (ideally this would be in the 5-10 second range). Remember, every second of time could introduce noise, movement or glare that gives your photo a subpar look. The key is to give your sensor enough light to capture what you are seeing, but no more than necessary. The photo was properly exposed at 20 seconds, F/3.2 at ISO 1000.
Night photography can be extremely rewarding since it opens up many avenues for your creativity!