I happened to run across an interesting tutorial for Corel's software called Painter. No, I don't have the software. I found it as a result of some link or other. The tutorial tells you how to use Painter to design both simple and complex plaids. Go here to have a look.
You can downlowd a 30-day trial here to test it if you are curious. I'm kind of curious........ Too bad it costs so much money!