I'm going to start sounding like a shill for Flusser since this is my second straight post recommending "Dressing the Man", but he does include a chapter about pattern matching. The basics are:
same pattern, use different scales.
different patterns, use similar scales.
use patterns which contain a common color and/or pick up a color from another clothing item.
avoid multiple small-scale patterns.
Of course, I've vastly oversimplified the advice.
Andy Gilchrist of AskAndy fame also has information on his website and his CD, which I can also recommend.