That's a tough one to solve especially with a closed face helmet. Some of the anti-fog stuff you can buy helps a bit. Best thing I can suggest is to get moving before you close the shield. That way there is some air flow going across your glasses. Another alternative is to leave the shield down when you stop so your glasses stay the same temperature as the air in your helmet. It's hard to talk to someone though. If you went to a motocross style helmet with goggles there is a company that build your prescription into the goggle lens. I think they are called Pro-Vue.
The older I get the better I was; In fact now that I think about it I am a legend.
I had the same problem and it drove it me crazy. I tried all sorts of snake oils to keep the fog from forming- no luck. I tried masks; the best one I found was a rubber respirator with the filters removed. It worked but if I took it off when stopped, it was really cold to put back on. I finally just got contacts and it solved all my problems (vision related anyway). I know contacts aren't for everyone, but there's my experiance.
you just got some real good advice... i run the retired pop set up. contacts were my first choice. i do use a electric sheild set up when its really cold but i still have to concetrate on my breathing..as soon as you stop crack it and keep it closed and dont breathe hard while riding. good luck with your delimma... you just got what i learned after 20 years.
Where I sled: Wisconsin, Michigan and northern Ohio
I wear glasses and heve the same problem. I now wear a moot X style helmet, a belaclava and a pair of smith Turbo googles. The Turbo's have a 2 speed fan built into them and pull air from the bottom of the googles through the top and work awesume. No more fogging of my glasses I beleive you can get them through Dennis Kirk.
I got a Ski-doo modular 2 helmet last year and that solved about 90% of the glasses fogging problem. I didn't put the visor down until moving - leave it cracked for the first minute or two then close all the way. Sometimes I will still need to crack the visor a little to keep the glasses from fogging. The helmet is heavy and expensive but is the best I have used for running with glasses.
That said I am going back to contacts for riding as much as I can. It is so much better with contacts - my problem is my eyes don't always tolerate them...