Mine isn't due until June, by which time DD will have just turned 4. I've spent the past year getting her ready for this, talking about how lovely it is for her friends at nursery when they have a baby bro/sis, focusing on how good it is for them, never talking about the baby. Fortunately she now has a best friend living next door who has a baby brother, so she has now decided she definitely needs to get herself one of those (not sure if a sis will be acceptable!).
I'm approaching this looking at all the positives - she's going to be old enough to be mummy's little helper, she's also old enough to be quite a different 'creature' to a baby and therefore require different things of me (I'm not sure how true that is, but it feels like it makes sense in my brain), and so not competing for the same things as a baby.
I'm also making the pregnancy very much about her. When we tell people in her presence, we won't be saying "we're having a baby" but "DD is going to be a big sister". I'll ask people to make a big fuss out of her, for 'her news'. I'll keep emphasizing the things about this that are going to be good for her - you get a baby to play with with, you can choose some clothes for it which is just like shopping for dolly clothes, etc, etc.
I don't know if these are going to work, having not done most of them yet, but they're my suggestions for this one.