Almost a year ago, I made my one and only attempt so far at star trails and learned a few things. I tried it at a relatively dark spot but apparently not dark enough. Here's an essentially unprocessed shot:
The settings I used for this shot were f/22, ISO 100, 10 minutes.
The lesson I learned from this is that I really need to try this either in the middle of nowhere or at an official Dark Skies park/preserve (
http://en.wikipedia.org/wiki/Dark-sky_preserve). As you can see from the image, there was a fair bit of light pollution, even though I was fairly far from a significant source of light.
If you manage to find a good place to shoot, I have a couple other bits of advice, both concerning the battery. Lithium batteries are less effective when cold. You may want to find a way to keep the battery warm so it'll last longer. The second important thing to remember is that some digital cameras spend a significant amount of time processing long exposure photos after the shutter closes. My camera (Sony a330) has a 1:1 ratio of exposure time:processing time. So, if I left the shutter open for 10 minutes, my camera would spend an additional 10 minutes to process the shot. If the battery dies during the processing time, the entire shot is lost. Maybe do a test shot indoors to get a rough idea of how much battery life is used for a shot of that duration.
One final note: If you have noise reduction, turn it off when using a tripod.
Other than that, I hope you get a clear night (that doesn't become overcast). I look forward to seeing the results and hearing about anything you learn from the experience.