Tips for nighttime long exposures
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Pay attention to ambient light. Except in the case of very dark, moonless nights, there is almost always going to be some ambient light in your scene. Your eye is not going to interpret that light in the same way as your camera does, so it's helpful to become familiar with how the different types of light are going to affect your scene. A full moon, for example, will bathe your whole scene in a soft light. Light pollution from a nearby city, on the other hand, may add an unpleasant cast to your horizon or make it difficult for you to capture stars.
Know your ISO. With daytime long exposure photography, you generally want to keep your ISO as low as possible. This is also true for nighttime long exposures, but give yourself a little more flexibility. You will get some additional noise at the higher ISOs, and long exposures can also create noise. But depending on how much ambient light is in your scene you may not be able to use those small ISOs you use during the day. Experiment with different ISOs until you get an exposure you're happy with, and generally try to keep that ISO as low as you can.
Focus to infinity. As when shooting with an ND filter, it can be tough to get focus and composition right when you're shooting in dark conditions. Since you're shooting digital, you can throw away a few frames to make sure you get this right. Focus, compose, and turn up your ISO as high as it will go. Take whatever exposure your camera tells you to and then check it on your LCD screen for focus and composition. Make adjustments if necessary, then turn down your ISO and do some real shooting.
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