I have had a bit of staining as well and cant get rid of it. Once I noticed it starting I put a micro fribe rag on the seat first and it hasn't become any worse.
Hopefully someone will reply that has had success.
I saw a stain from my bag getting started and immediately pulled the pad causing it off. It took a fair amount of soapy sponge and elbow grease to remove it. My guess is Simple Green would have done the trick, but I didn't have any handy. I should pick some up though, because the white part of my main seat is more of an "eggshell" white now.