My daughter was born when my son was 3 years 3 months old. While I was pregnant we did some role-playing with him or his stuffed animals pretending to be the baby. I also introduced him to any young babies I could find and any of his friends who had siblings, older or younger. We talked and read books and watched videos. I made a photo album of him as a baby so he could see that once upon a time we did all the same things for him.
But I have to confess that it was still quite a shock to him to see how much of my time was suddenly taken up by a yowling stranger. Fortunately, she didn't mind him cuddling with me as she nursed. He wanted me to treat him like a baby too a lot at first. I tried to do as many things all together as possible, but he quickly got tired of the nurse/diaper/sleep cycle. Although one of my friends with two daughters said that her oldest loved helping her care for the new baby. Having a baby carrier was helpful so we could eventually go on walks and such. Whenever I can, I take a few minutes of "All About Us" time and ask him what he wants to do.
Now, six months later, he loves her and calls her "my baby" but still struggles with wanting my undivided attention. Having other people around more, like my husband or DS's cousins, helps my little extrovert a lot. When my husband is home we switch off so that I can have some one-on-one time with DS. I tell him that he and his sister are special friends and how much she loves him. He's the only one who can always make her laugh, and he likes that. He's growing into the big brother role and likes showing her how to do things.
ETA: Also, practicing what he could say to the frequent variations of "Do you like your new sister?" Or "Are you a good helper/big brother" would have been helpful for him, I think. I didn't mind his honesty, but I think he may have been surprised at the reactions to the "wrong" answer. Something I read compared it to how I'd feel if DH had suddenly brought home a new wife because he liked his first one so much.
