Choose the proper ISO
Here in the day of digital, this is a simple task that can be achieved through trial and error. Each camera model react differently during low-light long exposures, so start at ISO 800 and adjust accordingly. You're going to get some noise, but it's best to try and avoid the obnoxious, brightly-colored pixel noise often associated with digital cameras and extremely long exposures. To help, you can try the dark frame technique described here by astronomical photographer, Jerry Lodriguss.