Boring topic? yes I agree, but it is vitally important to store your sails correctly. I myself years ago used to just put sails wherever they would fit, and in whatever condition that they happened to be shoved in the bag, now though after seeing some sails and canvas, I advise otherwise.
Not just used sails by the way too, I recently had a client whose brand new spinnaker went mouldy in the bag before he had a chance to even use it. In this case I told him that I can only assume that the sail was built in humid conditions, and then stored on the boat, again in a wet, hot, humid environment, that was all it took for the mould to grow in the seams where the double sided tape was yummy food for mould.
I'm seeing a lot more full sets of racing sails come in post a regatta or ocean race so that they can be washed of all salt and other grime that the race crew put on them as they were trampled on the cabin sole during the race.
Post the recent 5.5 World Championship, we had two sets of sails to wash and dry so that they were ready for their shipping back Europe, smart little Europeans.
So to finish this most boring of topics, store your sails clean, dry and in an environment that is also clean dry and well ventilated. All these same rules apply for canvas items too.