Well, this is just an echo of the other posters but this is what I did last year when expecting #4. My 4 YO, 3 YO, and 2 YO went shopping and bought something for the baby with Grandma. They were so excited to go shopping with Grandma and pick out a present for the new baby.
I also got "A Child is Born" and we looked often at the pictures of the baby's development and when you could feel the baby move from outside my belly I let them touch it. "How I was born" is by the same author and it's child-focused.
I moved the carseats around how they would be moved so that there wouldn't be added upset when the baby comes. I am pregnant with #5 so I'm in the process of doing this again. Only this means moving my 3 YO back a bench and will make it more difficult for me to strap her in, and she'll be ALONE until my 18 month old is two, but I'm going to go ahead and do it anyway. (I'll have a Britax Blvd and a Peg Perego infant seat on the first bench, two pink Britax Regents on row two, and two pink Britax Regents on row three! Crazy! LOL Oh, and a booster seat on row four for when we have my niece with us, that makes 7 seats in all. haha
Before I digress further, when I had #4 I bought a little gift for each of the girls and packed it in my suitcase and when they came up the the hospital to see me and the baby, I gave the gifts to them 'from the baby' and we had the 'sibling cake' from the hospital delivered to us and had a birthday party! I'm not planning a hospital birth this time so I'm not sure what I'm going to do. If I have an emergency delivery in a hospital it will be an hour away so I doubt my girls would be coming to the hospital that time.
Anyway, happy birth. My sister's girls were three years apart and it was really hard for the 3 YO to get used to sharing ect. But if he's been in daycare or is used to sharing with friends, he'll probably adjust better then she did.
S., SAHM of 4 girls ages 1-5 and one on the way!