When my second son was born the transition was very smooth. My oldest son loved babies and was so excited to meet his new brother. He helped bring me diapers when I needed them and for a while even thought that he would like to help change the diapers. Not long after that he decided that "green poop is yucky" and wanted nothing more to do with diapers of any kind, including his own, and he quickly potty trained.
The situation is changed this time around. My youngest son loves to be the center of attention and does not like anything to interfere with his cuddle time. We have tried slowly getting him used to the idea that he is going to have a baby brother or sister in the near future. I have held other babies and talked to him about babies. He has screamed and pushed the babies off my lap, insisting on completely, undivided attention.
Every child is different. Just because you are not a first time parent does not mean that you will automatically know what you are doing.
Read more: Becoming a Big Brother or Sister: Preparing your Toddler for a Sibling