I called Swarovski customer service and asked what they recommended. They told me "Parker's Perfect" anti-fog products. I bought 100 towelettes for about $15.00, and carry one or two when hunting in case I need to retreat the lenses. Works well so far.
A friend told me that he uses RainX black, its for the inside of windshields. Says it works great. I haven't tried it yet. However, everything else I've bought in stores and tried has failed.