Assuming that you've saved most of the baby essentials from your first child, these are the things you might want to prepare for your second child. Since Olive was getting her own room, we had to buy a new crib, dresser, glider, bookcase, etc. for her nursery. I also needed baby bottles, nursing pads, milk storage bags, and pump parts right away, but didn't have them prepared since Olive decided to arrive so early. Thanks goodness for Diapers.com and their free overnight delivery! Had I known that I was going to have a VBAC, I also would have prepared all the pain relief items beforehand as well.
Of course everyone's needs are going to be a little different, but I think this is a good general starting point. What did you need to buy for your second child?