We've successfully (nearly) trained ours that, if they do not want things broken or played with by The Small One, they must put them away EVERY time they are done playing with them AND in a location inacccessible or unknown to him. Mostly, we have not allowed "mine only" toys, opting for mandatory sharing of almost everything. This has worked out very well, and each child does have a group of items that we have allowed to be designated "special", which means that they do not have to share them. Once an item is dubbed "special" everyone knows that getting a turn with it is "by permission only, or else".
With four (soon to be five) children who are too young for things with small parts, it can be difficult for the older ones. There are a number of things they can only get out when the little ones are in bed. Same for me ... it's rare that I get an hour for sewing or stamping without a crowd trying to "help"!