The idea of BATNA (Best Alternative To A Negotiated Agreement) is about your ability to walk away from a deal. It is "the most advantageous alternative that a negotiating party can take if negotiations fail and an agreement cannot be made." This will dictate your success in any kind of negotiation.
As soon as you want something too badly, and the other party knows that, you will be taken advantage of. (This is also how Venture Capitalists operate, by the way.)
There are three ingredients needed to gain leverage (and therefore successfully negotiate):
Time (patience)
Information
Options (leverage)
If you have all 3, you can walk into negotiations with confidence.