B.M.
I agree with the other post about the doll, but also start getting her used to being independent. The posts I see where the oldest has the most trouble are when they have mom and dad 100% at their disposal - no other interests or life of their own- and then WHAM! diverted attention and they feel left out.
Can she take a class a couple days a week? Like a mommy and me thing or even a MDO where you drop her off somewhere? How about pre-school this fall? That way you're getting her used to having HER thing and you can also have a break once the baby comes. Also start working with her on playing independently for longer stretches of time - coloring or playdoh or whatnot so that she is used to that when you have to do something with the baby and then she won't feel 'banished' and instead will view it as it's HER time to play with the ...whatever it is....