Eric Sink has a recent blog post about negotiating. He makes the common sense observation:
In negotiation, the one thing that really strengthens your position is the ability to walk away from the deal.
My salary history over the last ten years bears out Eric’s observation. With every job change except one, I received a decent salary increase. The one exception was the job I accepted after having been laid off. I was not in a strong position to negotiate salary, and I took a big hit for it. It took me several years to get back to the salary I had before.