I wrote a quick piece a couple of years ago about what to consider when buying online. The rules haven't really changed during that time, but the process has gotten better. A few of the major players are getting better at dealing with difficulties and prices have stabilized a bit lower than before.

I get thousands of new people visiting the site each week and wanted to highlight the steps to getting a great pair of glasses at prices you never imagined.

Here are those steps again:

1. Get your eyes checked. This is important from a prescription strength and an eye health perspective. Keep a schedule and get this done annually -- yes, I've changed my tune a bit on this.
2. Get your full prescription. You paid for it. Make sure you leave with it. If you didn't bring it with you, give them a call. The law says they need to provide you with it.
3. Get your Pupillary Distance (PD). This isn't a part of your prescription by default. See here for some ideas on getting this measurement. It's important and it's not that difficult to get for a single vision prescription (bi-focal or multi-focals require a bit more expertise).
4. Get your frame measurements. There are a couple of good ways to do this. I liked my old frame style, so I knew that frames with similar measurements would likely end up being decent looking. If you're looking for something new, find a pair you like at the store and write down the measurements from the frame. In either case, the measurements are typically stamped inside the temple and/or on the bridge.
5. Find a frame. Hit the websites listed to the left and start browsing. You're looking for sizes as well as style here -- don't forget that! If the site offers a way to drill down to a temple size, for instance, use it! It will save you a ton of time. There is no reason to look at all of the 130mm temple-lengths when you need a 140mm.
6. Choose your lens options. I was oversold on "featherwates" and hi-index lenses for years. Depending on your prescription strength and the lens size for the frames you're looking at, you may be fine with the standard thickness lenses. I like anti-reflective lenses and I'm amazed at how much I like the photochromic lenses. With the low cost of these glasses, pick up a pair with sunglasses lenses. I don't buy lenses that don't include UV protection -- neither should you.
7. Order up! Use a credit card (not a debit card) to aid in unlikely event of problematic resolution issues. Pay a couple dollars extra for faster shipping from a number of the retailers.
8. Wait. This was the hardest part for me -- mostly because I was without a spare pair. It gets easier when you're not in that pinch.