I've worn glasses for 15 years (my eye shape and prescription aren't suited to contacts), so I've got a few ideas (regardless of fashion)
Either go early to your appointment, or go on a separate day (before your appointment) to browse at frames when your eyes aren't dilated. It's much easier to see what you're looking at.
If you already know what kind of prescription you'll have (bifocals, astigmatism, etc), then take that into consideration. For instance, I have a very strong astigmatism, so frameless glasses are out because I can't tolerate any twisting or rotation of the lens.
Try on both big frames and smaller frames. With smaller frames, you see the edge of the frame in your peripheral vision. You also have to tilt your head (instead of your eyes) to read a book. Some people find this annoying. Fortunately, the hipsters have made bigger lenses more fashionable (if that's your look). You can also mitigate seeing the frame by choosing more delicate frames and/or frameless or partially frameless.
Try on some of the funkier or statement-making frames. You never know what you'll love.
Also, consider how long you'll be wearing them. How quickly does your prescription shift, or will you get new glasses because you're finally tired of this pair? For me, glasses last about 3.5 years. Cost comes into this, too--how expensive will the lenses be?
Hope this helps, and have fun shopping!
