I've been ordering eyeglasses from Zenni Optical since January 2014. The process is very easy and after some trial and error you can find yourself saving a bundle or having different frames for every occasion, prescription sunglasses included! The very first thing you need to do is get your prescription. If you need prescription glasses your optomitrist should already have these on file, just ask them for a copy and if you haven't had your eyes tested in a while take some time to get it done because your prescription can change if your vision's not as good or it's been 1-2 years.
You're also going to need to know your Pupillary Distance (PD), thankfully Zenni has a guide on how to do this. The last part is a bit of trial and error, getting the correct measurements for your glasses. It took me two orders to work out which dimensions I needed, particularly for frame width and temple length. It's a bit more involved than visiting a local optomitrist where they will be able to fit the glasses for you and advise which frames will fit and if you've a big head like me you'll discover not all styles will fit, but you can filter the glasses on the site by dimensions which is really handy.
The order I just placed on the 23/12 cost me just USD $78.60 for four pairs of glasses, in my opinion a great deal.