Similar to Sue Adams, when something involves sharing one item, particularly food or something that needs to be split in half, I allow one child to split the item and the other child to choose which half they want. You have never seen such perfectly equal halves in all of your life :)
As for sharing other things, this is quite difficult. i have used consequences for not sharing, like having something taken away. I've taken the item away from all of the children and given the item a "time out". But, as the children have gotten older, I have made statements like, "I'm not going to make you share this item, but when you want to borrow something, I will not make your sister/brother share either". This also works at times, particularly when I mention specific items that they like to borrow from each other.
I guess it really depends on the situation.