The real key, as others have mentioned countless times before, is to print this on the ticket. That way people know several weeks ahead of time, and they will be refreshed early that day too when they tear that day's ticket off the rest of the bunch before they leave their home.
The real question is... why don't they market the wearing of gold for every home game? It's as simple as saying it at the end of every ad for an upcoming home game (any sport). "Join us at [insert venue] next week when your Golden Gophers host the [whomevers] at [whatever time]. And as always, come early, be loud, and wear gold."
If the U is serious about these Gold-outs, they need to create a situation where most people will be wearing gold to the game anyway. See: Penn States white out. They can get away with it easier because most people will wear white no matter if its a white out or not.
They also can get away with it because, let's face it: who doesn't own something white? From a clothing standpoint, I have noticed over the past four or so years that the Coffman Bookstore, Goldy's locker room, and online apparel sites have done a better job of putting more gold options out there, rather than almost exclusively maroon things - especially in the area of cold-weather items. That is an important step.