I've tried two products that seem to help quite a bit with this. One is Fog Zero and the other is Clarity DeFog-it. Both are safe for sport optics.
Clarity De Fog it is hands down the best I've used.
I put it on glasses and SCBA masks at teh fire dept and goggles... we fog up quick in fire situations... its the only thing that comes close to being reliable.... A shame its as high as it is, but I"m going to buy another 50 or so dollars worth of it shortly... running low.