Getting to Yes: Negotiating Agreement Without Giving In, by Roger Fisher and William Ury

A lot of it is common sense, but it's surprisingly helpful to have the common sense explained in concrete terms.

My two favorite parts were the suggestion to appeal to an independent objective standard, and the advice to always understand your BATNA—Best Alternative to a Negotiated Agreement. Knowing your BATNA and (if possible) the other party's BATNA can be a real help. Also, the authors mention that sometimes, you're negotiating with the wrong person. Find out who is really the party you need to negotiate with.

Also, sometimes the why behind each side's negotiating points can help reveal a solution that satisfies both. The illustration of this is of two people negotiating over who gets an orange, with neither person realizing that one wants it for the juice and the other wants it to make a cake with the peel. A poor negotiation leaves each person disatistfied with half an orange; a good negotiation gives each person what he wants.