I use a heavy plastic bag (one for each Brooks) and broccoli rubber bands. (y'know, the thick blue ones around bunches of broccoli) When I'm on the bike, I don't worry about the wet. But I proofhide the bejeebers out of the top and bottom of my Brooks saddles, so i'm hoping they're well protected while I ride. And there's also my butt protecting the topsides...
I also use the broccoli bands as "parking brakes."
They work great!
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson