I put a breather filter on my breather hose, and like Jiminok said - it gets some oil residue on it each ride. For now I have been putting a piece of shop rag around the filter with a rubber band, and changing it each ride. The professional answer is to install an oil collector that catches the oil vapor, yet still allows the crankcse to breath. Oil collectors are made for street bikes and car engines, and can be fitted to the DS, but I've not found one yet that suits me just right in my budget. I plan to make something in the future, but the rag works fine and is out of sight anyway, so I'm in no hurry.
The Hondas have a collector I'm told - take a look under a 300EX or 400EX sometime and you should see it.