There's a good book about negotiating that I think people here should read, it's called "Secrets of power negotiating". I have a blog post that summarises it but I haven't written it up yet, although I hope to do this soon. It's very practical and includes many examples on how to negotiate, including the example you said (create new items of negotiation to move the focus of another one).
One of the great benefits playing a MUD back in the day gave me was the ability to negotiate a lot when trading stuff, so I got a feel for when to push, when to take the offer, when a negotiation just wasn't working out, etc etc. That said, this book has about 90% of everything you need to know.
I don't really like self-help/improvement books and the like, but this was just no-nonsense and straight to the point.
