Protecting surface that has small surface rust already?

If you do not want to do as much as rabius75 suggested (It's certainly the best thing) I would at least sand as much rust off as possible and cover it with rattle can rust preventive paint.

If the car is going to be parked outside go to your local outlet store and pick up a plastic tarp and cover the ground under the car so moisture is not coming up from the ground. You don't want it to big so it acts as a water collector and funnels it under the car. Then cover the car with a cover that breaths. A plastic pail on the hood, roof and trunk to create an air space is a good idea to.