I'm studying for the foreign service oral exam now. I waffle between being hella nervous about it to thinking that there really is nothing to study at all. Which is sort of true. Basically you just have to use the communications, strategy and negotiation skills you already have......but you could always brush up on them.
I am reading Getting to Yes, which sounds like a crappy book but which is actually quite useful, especially the advice that we should be looking behind the other side's negotiating position and instead see what interests brought them to that position. That allows for a lot more creativity in terms of finding a solution that works for your and the other side.