This tip from Mercedes-Benz suggests spraying Zinc Dust Spray Paint onto brake rotor surfaces prior to parking/storing any of their models for extended periods.
Apply the zinc dust spray paint to disc brake rotor surfaces at various points on the outside and inside surfaces. By turning the wheels or moving the vehicle a few yards, the dust will spread out across the entire friction surfaces. The zinc is essential in protecting discs from corrosion and preventing shuddering and squeaking.
The Zinc Dust Spray Paint is available as P/N A 000 986 82 42 09.