There are 2 types of covers - bubble type and foam type. From what I've read, the bubble type will bunch up and not do the job; the foam will stay put.
There are some people at rhtubs that swear by the foam type - They say it stays put and really "seals" the water from the cover - there's no condensation on the cover at all.
Do a search on this site for a poster called "ZZTop" about a way he engineered to use the bubble type blanket effectively.