They sell anti-fog drops at most scuba shops. You put one or two drops on each lense, rub the drops in, dip the mask in the water, and you're good to go.
On our trip to Cancun, they offered free Try Scuba classes in the hotel swimming pool. My wife and kids went to it, and they said the instructor was using Palmolive Dish Soap on the goggles. She put a few drops in, had them rub it in, dunked them in the water a couple of times, and they were good to go. She said the secret was to NOT touch the lenses after rubbing the soap on. They swam in the pool underwater for around 10 minutes, and had no problems at all.