I got one of these, works great, and nice and cheap, goes on in 30 seconds comes off even faster and cleaned with a hose.
http://www.calcarcover.com/product.aspx?id=219&cid=78
I had generic wet okle from Costco before but they seemed to lose thier water proofing over time. I leave this one on all the time for mountain biking, boating and to make sure I don't bring grease from work onto my seats. If I go out on the town I can take it off and stuff it under the seat.
Mark