Read lots of stories about sharing to your child. Make up some of your own. Acknowledge that it is often hard to share but that sharing brings intrinsic rewards (praise by parents, fun with friends, etc.).
Whenever possible, have multiples of things (two similar trucks, two pens, two pads of paper, two dolls).
Hype it up whenever your child does share. Praise, praise, praise. Though your child should know this is expected behavior, she should feel good about doing it.