My son was 2, almost 3, when my daughter was born. I took him to all my doctors' appointments and we'd hear the baby in mommy's tummy go "woosh, woosh, woosh". I didn't think to get a doll, but I did point out nearly every baby we saw, so he'd understand what a baby was. I'm not sure he totally made the connection though, until we brought her home. He helped me pick out little toys and clothes before she was born, and helped show them to her afterwards. Several of the people who had gifts for the baby, thought to give him a little something too - which helped a ton. My husband also made a big deal the night before the two were introduced, of letting my son know he was a big brother now. Presented him with a plastic sword and shield so he could protect his sister. (Of couse they had to be taken away later, since my son seemed to be protecting her from the cats!)
Some other moms I know thought to take the older child out for a special lunch for just the two of them or a special day of fun just before the baby was born. Someone else mentioned specifically getting them a gift from the baby when the baby is born - I wish I'd thought of that one! If the baby arrives and immediately has a present for the older sibling it might help turn a positive swing on it.
Even with the stuff I did do, my son actually rejected ME when I came home from the hospital (partly I think he was pissed that I'd gone away to the hospital!), when I made a point of going to get him from his nap without the baby so he'd know I was still HIS mommy too. I told him I loved him and he pushed my face away with his hand and said "NO!" I tried again with the same response and had to leave the room. I hadn't slept in over 48 hours and was shattered to tears when he said that - broke my heart! It took him a few days before he would really come near the baby and I, but eventually he got curious. Nowadays he makes her laugh more than I do! It gets easier as the baby gets more and more interactive and now my son LOVES his little sister and tries to give ME advice on what she needs/wants when she cries! :-)