I use a neat little device from Scott, called the Mud Flap. You use it with rolloff system. It sticks to the lens of the goggles, then the rolloff film is slid under the mudflap, this keeps the water from getting under the film and smearring across the lens when you go for a pull. Of course you still need to water proff your helmet with duct tape and some foam.
From if you take a look at the photo that was taken on the 1st lap of a harescramble, I used the same goggles for the entire race, with no water problems.
http://cssphotodesign.com/gallery2/m...geViewsIndex=1
Here is a mud flap:
http://www.motorcycle-superstore.com...c=2&zmap=11634