+1 on the plastic bags when the bike is outside.

I agree with Clifton. Proofide is a conditioner to be used periodically.

To further the quote from the Brooks website:
"Apply a little Proofide to the finished side of the leather. Allow the Proofide to permeate until dry and then polish off. Proofide should be used several times during the breaking-in period and every 3-6 months thereafter. (on bicycles not fitted with mudguards, an initial application to the underside of the saddle will be beneficial. This needs not be polished off."

This I slightly disagree with. Proofide is a conditioner, not a polish. The top of your saddle is highly polished and dyed. For proper absorption, it's more important to apply periodically to the unfinished underside of the saddle regardless of fenders. (As well as a little on the top).