I just had my second born about 4 months ago and my girls are 23 months apart. To prepare my oldest, we read her books about being a big sister. Also, at the hospital when she first came to see her sister, I had her sit on the bed with me and then dad handed the baby to both of us. That way, when she first came to see mommy, we didn't have to deal with any shock of mommy holding someone else first, etc.
We also had the baby give her big sister a gift when she came home. It was a cabbage patch newborn doll with carrier and bottle and everything. That way, when mommy was feeding the baby, so was she, same with changing diapers, etc.
Both my husband and I also took a week off of work a couple of weeks before the due date, so that we could spend some quality time for the last time with just our oldest.