I got sick of working as an admin/tech on a mainly Windows (12:1) network, so I decider to start doing work on my own. It hasn't been to bad so far. I got in with one local magazine about six months ago, and they soon recommended me to others. It is great now, and I work with mostly Macs (10:1).
As far as suggestions, first get Apple's AppleCare technician training kit (you can find it at the Apple Store web site under AppleCare down at the bottom of the page for about $299.00). This gives you a ton of info that you wouldn't normally be able to get (even if you don't take the exams, you still get a ton of stuff). Second, while you are working on the course work provided, go to Apple's discussion pages and start looking for problems to try and solve (best training you can get for free). Third (as mentioned by someone else) know your client's software (Photoshop, Illustrator, QuarkXPress, Office, ATM Deluxe and Suitcase are some good examples).
For the work that I do, knowing servers (both print and file) helps quite a lot. Many print servers are based on Windows NT/2000, Sun Solaris and Silicon Graphics Irix, so knowing those systems is a plus. Some of the high end print server software to be aware of would be EDOX, Fiery, and Colorbus Cyclone. On the file server side their is AppleShare IP (5.x and 6.x), Mac OS X Server (1.x and 10.x), Xinet's K-AShare and Helios EtherShare.
That should give you a starting point. Even if you don't go out on your own, having that kind of knowledge base to put on a resume would be very helpful in getting work.