This tip is really just for peace of mind. If you have more than one child in the preschool category, this will be a good idea for you. Carry a few hot wheels type cars and little "men" around in your purse. These are only to be taken out for "waiting time", as in the doctor's office, etc. Let the kids play with them.
The tip is - no fighting is allowed because these toys belong to mom! The kids can't be territorial about them and disputes die very quickly. Try it, it really works. It would work for any type toy or book small enough and prized enough for you to carry around and only use on those special occasions.