I was going to post this the other day, but I went to go see Iron Man 2 and forgot about it till now.
This is one way we could make a more flexible spine for a manikin. It basically consists of several plastic vertebrae that snap fit together. It would have at its core a flexible metal rod that could bend and help it hold its pose.
This probably isn't the most practical way of doing this, but I figured I would post it to see what you guys think.