I think the most important bit about negotiation that I've learned and read is this: if you're not willing to walk away, your attempts to negotiate will fail. If you're not willing to say "sorry we can't come to a middle ground here" and leave the conversation with no regrets, you'll eventually cave in and lose. (This has been my biggest problem wrt salary negotiations. When I get an offer from a company that I like, I'm almost always unwilling to give it up. Having multiple offers on the table is the best alternative to this.)