its pretty simple, Ive just always put a plastic bag on it when its parked (two actually, one on top of another) and when riding I used to use plastic bags but then got a proper fitting Brooks rain cover that is comfortable to ride on.
Cool, this is kind of what I was getting after. I understand covering it while the bike is sitting, that's simple enough. But I was wondering what people do when they have to ride in the rain. I didn't think that any kind of cover would be comfortable to have in between the saddle and your butt, but, if the Brooks rain cover works for that then I know to keep it in mind.