If it's indoor, I'd suggest Dustop, which is a material made by Kimberly Clark. It's an updated version of the old flannel lined cotton, but won't allow dust to pass through (as flannel will) and won't absorb moisture as flannel will.
You can purchase large rubber mats for the garage - this will help hold moisture down under the car, too.
Make sure the car is clean (duster and quick detail) before putting the cover on.
If outdoor, I'd suggest Evolution 4 material or NOAH. Custom fit covers are a better investment. Buy the cable and lock kit to prevent cover theft.
Everything gets covered at my house except my wife's minivan, since it's never sitting still long enough to use a cover. They're a worthwhile investment.