DD1 is 2.8 and DD1 is 7 weeks. I was really worried as DD1 literally had people falling at her feet, she was that much the centre of everyones world.
We started off by saying she was going to have a sister and that mummy was looking after her. We encouraged her to come up with a nick name, "what should we call bump?" kind of thing. Then when in Tesco's (not every week though), I would ask her to chose something for baby - a baby grow or something. Occasionally we would ask her if she thought her sister liked this or that. It was all very every so often and only when it seemed ok. She was and is at nursery, so once we told them, they did a few bits on sisters.
We then changed routines, slightly that we thought would work when the baby came. For example, dad did bath time and washing hair. But I would dry her and help with PJ's. And either I or dad would do book. While the bath was running, my OH would stay upstairs, and DD1 and I would have a little chat/ read a book/ or have tickle time.
As she is at nursery all week, we changed drop offs and pick ups to ensure that one of us did the mornings and one the evenings, keeping to a regular time. We also chose a day each where we did both. Mine is Friday and it just meant, that we got one on one time with her. (at the moment its the girls time on Friday and she loves going for a little walk before we get into the car or feeding the birds in the nearby green etc)
Because of my OH's job weekends has always just been DD2 and me. So I made sure again, that there were routines then. Saturday morning we go to Tesco's shopping (yes, I did go the Saturday after DD2s birth and left her with a very tired daddy)etc.
I got her a little gift from DD2 and a card (she is obsessed by them) for when I had left the hospital.
We did read the story books about a baby coming and I basically just repeated some of the things in them. As she is also obsessed by stickers, I got her a sticker book (by Usborne I think) that does the baby is coming with stickers, which I put into the cot when she visited us in hospital so it did look like it came from DD2.
From talking to other parents at nursery, what seemed to have really worked is her age. Those with slightly younger children in the same class are having a nightmare with jealously. Don't get me wrong, DD2 can be clingly (mainly when tired) etc, but she loves her sister, and insists on helping with everything.
Oh and when DD2 was actually here, right away we got her involved. She fetches water to help when we changed DD2's nappy. If one of us is out of the room, she must let us know if DD2 wakes up or starts to cry. We read books to each other and then to DD2 and help DD2 come up with a new story.
Sorry this is long. But hope it helps.