Any Advice for Starting a Web Design Business?

stizoked asks: "Although we both have full time jobs, my wife and I have been doing a little web design/development on the side for some extra cash. Since we've started, we've built up a nice little client list, one big enough for us to consider getting a little more serious about pursuing it as a business. Does anyone have any advice or experience that we can use to dodge young and stupid mistakes? Any advice on some open source project management software or other software that makes running a small business a little easier?"