Okay, let's be honest we all know even adults sometimes have problems sharing. (and we all understand the concept.)
It's much easier for a young child to grasp the idea of taking turns.
So next time your child won't share try saying:
"You've been playing with Red Bear for a long time. it's Jonny's turn to play with Red Bear; let him play with Red Bear for a few minutes and then you can have Red Bear back "
Most of the time if you can convince the child the toy is coming back, they'll give it up. The only problem is you have to make sure that Red Bear actually does come back.
(If you don't all hell will break loose.)