Winter fly fishing is great, however, sometimes you have to ask yourself if it's really worth it. A few weeks ago as I drove down a local canyon I passed a couple of crazy friends who were out knee deep in the river. I glanced at the car's thermometer and saw 17 and I'm pretty sure that it wasn't set to Celsius. I asked them later how the fishing was and got the surprising answer that it was great. I still have a 20 degree rule but really it seems that the fish probably don't care what the air temperature is. If you can take it then go for it.
Whatever your rule and pain threshold though, make sure that you go out prepared to deal with winter elements.
Next, make sure your under-wader-wear is warm and flexible. Polarfleece type pants with a wicking layer underneath and good wool socks with a wicking sock liner are usually best. Use flexible thin layers up top and add or subtract layers to regulate your core temperature. Be sure to have good gloves, usually half-finger gloves made of Windstopper fabric are my choice. Make sure your hat is warm and covers your ears; you'll lose heat from your head faster than any other part of your body.
Lastly, it is best to wear wading boots with rubber soles or better yet, Aquastealth soles with studs. Ice and snow stick to felt and, unless you have a strange need to feel taller or to fit a leg workout into your day, felt soles are better left for warmer weather.
If cabin fever is setting in then get out there and fish, but be ready. I'll see you when the thermometer hits 20!