Wingbone is correct... liquid dish soap will work if applied in the fashion he outlined above.

Another method is to use an anti-fog spray. I was not able to find it at my area eyeglass offices, but I did find some at the paintball field. A company called 32 Degrees makes a non aresol spray for paintball mask lenses that works like a charm on eyeglasses. I imagine that any store that sells paintball supplies could get you a bottle if they didn't already have some on hand.

I've been using the same bottle, (pretty small), for the last 4 years and still have plenty left. It doesn't cost much and works even on the hottest of days afield.