I think the best way to look after canvas would be to clean it every now and then, keep it in the dustbag, in a closet or a storage box, away from sunlight and don't expose it to the harsh elements (sun, rain, snow, cold weather etc) for long periods of time if it can be avoided. I've got a Dior canvas bag that I bought in 2004 and it's still in really good condition in keeping it out of the sunlight, in its original dustbag and shopping bag.