It's because you are sweating.
Remove a few layers of clothing. (Only if at least one layer remains that is! 8) )
Dress so that on the briefing you are a little cold. That way during the game you won't sweat that much.
A lot also depends on the airflow. Don't wear a scarf like thing on your mouth, because that directs the breath up. Also don't wear anything right above the goggles that stops airflow.
If still fogging, switch to glasses.