So the rule is: if the girls fight over a toy, they take turns with it while I count to twelve for each turn. If they can't share it, the toy goes in time out. Its tedious for me, because I have to be on top of it all the time. A lot of toys go in time out.
I started doing that about three months ago, and I've found its successful in several ways. At the beginning, they'd get so caught up in the counting they'd get bored of sharing the toy and move on. The method does help teach them counting, and lately I've noticed they are enforcing the rules without me! They'll count and take turns all on their own. I'm so proud when they do that. Sometimes if they're getting angry with each other, one will run up to me and say "COUNT, mama!" Which is much better than hauling off and taking the toy or whacking their sister.
I highly recommend this method. I just made it up as I went along, but it really works for us.