1. Talk about the baby before it arrives. Even the youngest children can be told over and over about the baby inside Mama and how it's going to come out. They obviously won't totally understand, but they will understand some of it. Reading books about babies and pointing out babies in stores helps. Make sure older kids know what birth involves, especially if they're going to be present. (I've been surprised how many times one of my kids thought the baby was going to come out of my belly button!) They should know the sounds help, and it doesn't mean anything is wrong. Also, they should know that bleeding afterwards is normal.
2. Children mirror what they see and hear. If they see/hear you being fearful about the birth they will pick up on it, even if you think you're being discreet. Let them hear you talk about how cute the baby's going to be and how you're excited to have a new person in the family. As adults, we know there are negatives to a newborn, but keep most of that out of your conversation around kids.
3. Supervise little ones closely. A child younger than about 3 should just be supervised all the time with the baby. They don't understand what would hurt baby, so they may try to pick him up or put something in his mouth. Make sure they are taught how/where to touch baby instead of just saying, "Don't touch". If you're spending time with the both of them it will avoid them feeling neglected.
4. Get older children involved. I think older children just want to know they're not being replaced by the new baby. They need to know what their role in the family is going to be now. I have them make a present for the new baby. It allows us to talk about what the baby is going to be like and what they can do to help. They can bring you diapers, learn to make their own pb&j, or pick out what baby wears. I think sometimes parents are afraid to ask older children to help for fear they will feel put upon, but that is not my experience. If you try to do everything yourself it doesn't allow for bonding between siblings.