Car covers can be good if you buy the right one and use it properly.
Get a good quality waterproof one that has breather holes and is fully lined with that white furry soft stuff. Autotecnica are the best I've seen - they're made in Italy and about $110 for a large from most good auto parts stores or even some KMart stores etc. You'll need a large for a Commodore.
Yes, the car should be fairly clean and definately dry before putting it on. But if you always use the cover that shouldn't usually be a problem cos it keeps the car clean anyway. The soft lining generally protects it from scratches, but the key is to wash the cover itself regularly. I used to just hang it out on the clothesline and hose it (inside & out) every couple of weeks. Hope this helps...