Yup, singh is right. When I lived in SoCal I had a cover and I started to get surface runst in a few spots from it. It rains, moisture everywhere inside and out, then the sun comes out and turns the cover into a wet sauna slowly steaming your bike,
Still, with the cover gorilla posted you could put some small openings front, rear and sides at ground level, then a couple scoop covered vents on top like they use on boat covers. It gets you some ventilation and keeps the worst of the wet off the bike. Of course as soon as you start stacking all the improvements up you realize what you want is a real garage.
Maybe it's better to just wax the bike really well, touch up small rust as soon as you see it, and just replace the soft parts every couple of years as the rain eats them. I dunno, I'm spoiled. I have a real garage now and free covered parking at work.
Scott