I used to be known as the "Gap Girl" in college. (Basically I wore a lot of khakis and button-ups.) But a few years ago, when I started Being Spiffy, I wanted to branch out and find my own style. Now I'm a huge advocate of publicly displaying your personality through your clothing. One way I show my personality isn't just by mixing patterns (stripes and florals, anyone?), but matching patterns as well.
Case in point? Polka dots. I adore polka dots. Small ones, preferably. But when matching patterns, there needs to be a size difference. The polka dots on my skirt are about 1/4 inch, while those on my shirt are 3/4 inch. Over double the size. Because the skirt pattern is so small, it almost becomes a neutral that can be paired with anything.
After size comes color. If you match patterns, don't go cuckoo crazy on the colors. You'll end up looking like Bozo the Clown. Instead, choose a neutral and a bright like one piece in camel and the other in neon pink. Or two extremely complimentary colors like coral and blush.
The key in any outfit is balance, balance, balance. Size and scale combined with coordinating colors will lead to a simply smashing outfit.