The first thing you'll want to take into account is the shape of your face. If you're unsure what your face shape is, ask someone else. The receptionist, nurse, or eye doctor in your optometrist's office can be very helpful in this aspect - they've done this before! Once you have an idea of your face shape, you want to choose a frame that contrasts your shape. So, if you have a round face, stay away from a round frame, they will just make your face appear even rounder. Go for angular shapes with more defined corners. If you have a square or angular face, choose something with a gentler shape to it, perhaps an oval look will suit you. Don't be afraid to ask the employees for help. They are very experienced in this and can be a valuable tool as they will give you their honest opinion.
You also need to be aware of where the glasses are sitting on your face. Your eyes should be centered in the frame to avoid looking like they don't fit correctly. Your eyebrows should align with the top edge of the frame. If the frames are above your brows, try a shorter frame, if you have high brows, try something in a taller style. There should be a little bit of space between the sides of the frame and your face. If the sides of the frame are touching your face, it can appear like the frame is too small or that you've gained weight in the face (and who needs that?)
As if there wasn't enough decisions to make already, you need to decide whether you want a plastic or metal frame. There are good and bad points with both. Metal frames can be a little heavier but are more adjustable for a more custom fit. While plastic frames, on the other hand, are very lightweight but can't be adjusted much. I, personally, prefer plastic. They come in more colors, and I like the fact that they are lightweight as my glasses are always sliding down. Also, I tend to bend or twist metal frames out of whack!
Another thing you may want to think about is your wardrobe. If most of your clothes are in the brown family, a black frame probably isn't the way to go because it may end up looking too sharp or heavy. Your spectacle frames don't need to match every outfit you own, but if your clothes tend to be from one color family, a complementary frame color will look more natural. Even though you may really like those hot pink frames, if you usually wear red or navy, they could end up looking costume-like! When deciding on a color, you can opt for one that complements your eye color and really makes them stand out. If you have blue eyes, try a blue or brown frame and notice what they do for the color of your eyes. Also, when choosing a color, decide whether or not you want to make a statement with your glasses or if you would rather have them blend in and look more like a part of your features. If you're generally the type that doesn't like their clothing to stand out and tend to be more conservative, you might not want to go crazy and pick up a wild color on a whim.
If you're the type of person who likes to take a poll from coworkers or friends before making a decision, many optometrist's offices will allow you to take a selection of frames home with you for a time. This way, you can get lots of opinions and perspectives and you can also wear them around the house to see if they will be comfortable after hours of wear. There are also places where you can return your frames within a certain amount of time if you don't like them, and continue returning them until you find a pair you are comfortable with.