Go Classic

Really, these things are too expensive to blow a lot of money on something that isn't going to be in style in a year.  If money isn't a concern for you, then go ahead, but these are tough economic times for everyone and you shouldn't be spending with reckless disregard.

This is another reason why it's important to go to a lot of different places when you're on the hunt.  You get a better sense of what's in style, you can talk to shop clerks and get their opinions on what's coming in and what's going out of style.  They should have some idea of how their stock has changed.

Keep to styles that have been around for 50 years or more (i.e. black lacquered frames of some sort) or simple wire ones, which never really go out of style, but are also never really in style; they're an unobtrusive baseline.

Take Someone With You

Do not shop alone.  You can't trust the word of your optometrists or shop clerks, not because they're malicious and greedy—though they might be—but because they're not interested in what you look like.  Take a girlfriend, sister, well-meaning postal worker or friend—preferably a female one—with you to get their opinion.  It's always good to have a second opinion and you might not notice that you look like a tool in a pair of Buddy Hollys.

For that reason it's also important that you take an honest person with you; brutally honest is best.  Some people's faces aren't meant for certain styles, either because they're too wide for your skinny head or they call too much attention to an unflattering but normally overlooked feature of your face.

Fashionable Four-Eyes

Glasses can be an excellent fashion accessory.  You'll anger everyone you know who wears them out of necessity by fronting like you have weak vision, but that will pass.  They can either tie an outfit together or stick out like a sore thumb when they don't work, so be vigilant when you're shopping.