I would use a social story approach and talk about what’s going to happen and what their role will be as well as all the things they’ll get to do when the baby is old enough

As others already said try to get your kids as much as possible involved by the preparation of the coming and the care for the baby. There some nice books on the market about preparing your kids when mummy is pregnant and what is going to happen. We had one from Usborne (http://www.goodreads.com/book/show/228143.Usborne_First_Experiences_The_New_Baby.
We made a wee box for my daughter with special activities for the moments I was breastfeeding my son.

We brought our daughter a new doll with nappy bag and big sister books as a present from the baby. She was in love for the moment she seen him

I think getting them involved with the new baby is a huge help to them. Also we have to remember it’s a big adjustment for their little brains to understand so we have to be patient. Easier said than done though I know!

Include them in everything to do with the new arrival and also make them feel special and give them quality time too.

Include them in as much things todo with the baby!

My daughter was excited as soon as she found out she was going to have a baby brother or sister. I was a bit worried she would go when she met her sister or when we came home. Try to make them feel included through the pregnancy like buying clothes and jelping set up room or maybe even helping with names. Tips that i got from friends for when the baby arrives were to make a card for the baby, buy the baby a speacial gift and get the sibling a speacial gift from the baby. When you come encourage as much as you can to get the sibling to help out and praise them with what a such great job they are doing.