My dd is also bit older - just 2 - and will have a new sibling in the autumn. she sussed it before we told her as we have a few pregnant friends so she worked out herself what was in my fat belly.
We have a couple of really nice books about waiting for baby and then the baby having arrived. She got a doctor's kit for her birthday because she's fascinated by stethoscopes etc at doc's appointments and I wanted to make that side of things fun not scary.
She has a doll she loves mothering - pretending to change nappies and even breastfeeding!! She's got a toy buggy which she loves and I've got her a dolly sling too.
We know a young baby so we talk about him a lot (that he can't walk yet, that he sleeps alot and only wears pyjamas). Also she sees me cuddling him which she hated at first but it's getting better. She loves cuddling him herself though and is generally quite besotted by him.
We try and refer to it as our baby (as in hers as well as mummy's and daddy's) and have started to talk about what it might say when it comes out (Wah Wah Wah) and what it might do all day.
She's very sweet about it, kisses my tummy and sticks precious Peppa Pig stickers on it "for baby"! But I'm sure she'll find it a bit difficult when the competition arrives.
The books we've got are lovely but are in her room where she's asleep. If anyone wants the titles I'll post again tomorrow.
I've read it's important not to call the older sibling "a big girl/boy now" and insist they behave as such - that it's best to let them continue to be the baby as well if they seem to want to - they won't want to for long. In other words that it's good not to change how you treat them.
Interested to hear other people's thoughts and approaches. It must be quite a shock for a first child when the second one comes along.