You can either take one long exposure, or blend several 30 second exposures together. I did the second method, and here's how. Set up your camera where you want it, full manual mode, and then set your exposure to 30 seconds. If I remember right I had my iso up somewhat high so that it grabbed enough light from the stars. Just take one 30 second exposure, and see what it looks like. If it's the right lighting, you're good to go to take your long exposures. If not, change your F-stop until it is. Like he said, higher F-stop=less light through your aperture. once you get it figured out, just take 30 second exposures for however long you want, then blend the photos together in Photoshop, with "lighten" as your blend mode. Here's my result, this was over a 30 minute period, so it's 60 pictures blended together. The link below is to a video I made with the 60 images, another cool thing to do.
Edit: found the exif info.
exp: 30 secs
Aperture f/4.0
Focal Length 18 mm
ISO Speed 1600
http://www.flickr.com/photos/53511030@N08/5042940659/