Like Day Tripper, I've been wearing eyeglasses since I was a tyke. I'm at the point where wearing prescription lenses costs a fair amount of cash, since I wear graduated lenses, aka "bifocals without lines." I also have a high prescription, so I buy very expensive, but thin and light lenses so that I don't look like I'm wearing Coke bottles. It's a drag, but when you have to wear specs all the time, you deal.
I tried to play it cheaper and get a sturdy pair of aviator-style prescription sunglasses to cycle in without the very expensive lenses, but those really did look like bottles on my face. I finally bought a pair of Bolle Vigilante glasses for about $30 from either Nashbar or Performance (don't remember, but they're the same company now), and I got the insert as well. I had a single vision lightweight, but not super-thin plastic lens installed by my optician, and I use them all the time. The total cost for the sunglasses with inserts and lenses was about $180, about $50 - $75 more than my ordinary prescription sunglasses.
The good things:
* they help my peripheral vision by blocking glare from the edges
* the sunglasses themselves were relatively cheap, enough so that I bought a second pair to have as a backup. (I follow Grant Peterson's philosophy that says, if you find something you really like, buy a lifetime supply.)
* they don't fog much worse than my aviators did. I know this, because I just now rode to work in 17 degree weather.
The bad things:
* they're too wide for my skinny face. I would have been better off buying the narrower Parole glasses that use the same insert. It looks like I'm preparing to go into a radiation zone when I put them on.
* the case that comes with the glasses isn't sealed from the elements. That's not good if you're not real picky about the way you throw things into your pannier -- you want something that will keep your glasses from scratching if your Cheerios spill all over your bag. So I paid another $20 to buy a zipper-closing case and cleaning liquid and cloth from Sunglass Hut to put them in.
I don't have any problem keeping them clean, but I'm persnickity about keeping my glasses clean anyway.
Another thing that works for me is to have a supply of one-day contact lenses that you use for cycling. You can then wear any sunglasses you like. I don't do this for commuting to work since I can't see up close as well with contacts as with my progressive lenses, but riding with ordinary sunglasses is great. So I also picked up a pair of Coyote lenses from Nashbar for about $20 on sale. (Our Adventure Cycling friends want $50 for 'em, hmmmm.)
Good luck to you.