Chances are if you store your car in a garage the main problem you have is static air and damp rising from the floor. If you use an indoor car cover then this will keep the dust and dirt off it and protect it from little accidental prangs and scratches while brushing past it. The car cover must be breathable as damp that rises from the floor must be able to escape from underneith the car cover.
We have found the best solutions are to seal the floor with a concrete paint and or fit a plastic membrane. This can be bought from any hardware store. Look for the type that goes under laminate flooring as this is quite thick and will stand the test of time.
This will help to keep the damp out and your car pristine.