>How would I establish a client base, that is, how would I attract clients?
>Are magazine ads the best way to start?

IMO the best way to find clients in this business, particularly when you're just starting out, is via the people you know or have done any kind of work for in the past. Getting that first client is the trick, from there you can build up your business on word of mouth referrals.

Print advertising is probably low on the list of effective ways to attract clients as an independent developer, at least until you have established some name recognition for your business. By way of example, when I first stated out as an independent I place a small ad in the yellow pages; seven years later I can trace only one client to that ad, but I have gotten numerous phone calls from people wanting to know if I sold this or that piece of obscure software, or if I would come out to their house and figure out what was wrong with their PC. So that wasn't too effective, overall. On the other hand, if you have a particular target market in mind, e.g. a particular industry or a particular type of product you're going to develop, then a print ad in a publication which targets that audience might be effective.

And of course, don't forget the web!