K.M.
I thought the book "Siblings Without Rivalry" was fantastic. The authors- Abraham & Mazlish I think- have another equally great book "How to Talk So Kids Will Listen and Listen So Kids Will Talk."
What helps with my 3 yo dd and 5 yo ds is staying close and catching it when a conflict first starts brewing. Whether I catch it early or not (because I don't always), just commenting non-judgmentally on the situation: something like "You both really want to play with that car at the same time." Sometimes just from that they come up with their own solution, otherwise I'll offer solutions like "Hey, let's get you another car" or an idea for me joining them or a different activity for one or both, etc. I never "make" my kids share- we take each situation individually and work out something agreeable to both. The result is they feel "safe" with their stuff and much less likely to lay claim to things and fight just because it's "MINE." Actually, my 3 yo is recently experimenting with the idea of possession and will sometimes run around the house with something big brother has requested with a devilish grin and her little fingers up saying "just one more minute." My 5 yo gets frustrated yet is able to grasp when I tell him she's little and just learning about power and negotiating and having stuff and it might just be better to find something else to do for a bit and she will quickly lose interest in the "game" and drop the thing. Then we can share a secret smile about the antics of his sweet little sister as she runs about the house giggling with the current object of their affection.
IMO, the whole practice of sharing between children has quite unnaturally become something that does not reflect at all how adults negotiate the use of things. We don't go up to something another adult is "playing" with and make them give it to us (either immediately or within a certain time frame) just because we want it. We say, "Hey, can I see that when your done?" or "How much longer are you going to be using that? I'd like to use it." Also, punishment does nothing constructive if your objective is helping children learn tools to get along with other people and will very much contribute to adversarial relationships all the way around. Same with rewards or progress charts and such- for more on that check out the book "Punished by Rewards" or "Unconditional Parenting" by Alfie Kohn (Can you tell I'm a book girl? :)