Preparing for the baby

This morning when I dropped R at school her teacher called me and told me she had been acting strange lately. Apparently she has been cranky and almost aggressive towards the other kids, she pushes them and she doesn’t want to share the toys. The teacher says this is not at all like her. I asked her if she thought this had something to do with the new baby and she said it’s very possible and that it has been happening more since my belly started showing.

I really don’t know what to do about it. I always thought that a sibling was the best thing I could give R, and I still think so. But how do I explain to her that this baby is not a threat to her? How do I prepare her?
In a way I am glad she is having these reactions now, when I still have plenty of time to work with her and prepare her before the baby is actually here. It hurts me to see my baby hurting, to feel that she’s insecure and confused. I wish I could protect her from all these feelings but I know I can’t. This is part of her growing up and I can’t do it for her.

So, those of you with more than one child, how did you prepare your older child/children for the new baby? Any tips?