In foggy conditions, I try to wear my glasses a bit low on my nose. Then if they fog up while I'm stopped, I can kind of peer over the top of them until they clear again. Of course, this only works if you can see reasonably well without them...

I also sometimes take them off while waiting at a light (just pay attention to the lights the other way so you know when to put them back on). This doesn't usually clear them if they're already foggy, but it can stop them from fogging up.