I must say that as time drew nearer for the birth of my second child, I was really trying to prepare my son for the arrival of his new little sister. The day that my daughter was born, my son kept saying mommy I want to hold her; can I sleep in the bed with you too mommy. I want to share with you a few tips that will help the older sibling prepare him/herself so that it won't be too hard once they see the new baby.
Allow the older siblings to help you with the baby. What I mean by this is, when you get ready to change the baby, let the other child bring the diapers, baby powder, or whatever it is that you need. I know personally for me, I allowed my son to bring me sarah's clean clothes, her bottle, and diapers. As time went by, he started to feel like a big boy and being around the new baby didn't get to him as much. Now, I must say that that still didn't stop him from getting jealous.
Make time just for the older sibling. During the first couple of months, there will be a lot of attention paid to the newborn, so you have to be creative with this one. With my two little ones, when Sarah would go down for her nap, I would watch a movie with Ishmael, or play his favorite game with him. Sometimes, that didn't work, because how about little missy, didn't want to stop eating (I was still nursing her), so I would play with ishmael in between playing little games with Sarah while she ate.
Let the older sibling know that he/she is loved. This may seem like a given, but little children still tend to get jealous of the new baby. Even before the baby is born, keep remind them of how much mommy and daddy love them and that the new baby is not going to take their place. I know with mines, I consistently allowed ishmael to feel my stomach when Sarah would be kicking, and I would tell him that "she was kicking trying to come out so she could play with him". "Another thing I used to tell him is that he's going to be a BIG brother and little sisters love their big brother because he has to be the one to protect her.
I really hope this get you on the right path as to preparing your older siblings for the new baby!