I think one of the most important things you can do is tell them how much you love them and that you and the baby need their help. Make them feel as included as possible, and then they won't get as jealous. When the baby is born, make sure to have special time set aside just for them and make sure to ask for their help.. to grab a diaper or pick up the babies bottle. My mom was always really good at involving all of us, and it made my younger siblings offer constantly to help
A toddler will always get jealous if a new baby comes. You need to explain to them that if they become big brother or sister, they become like mom who will take good care of the younger baby. It is good to give them resposibility even in their younger years.