Your strongest negotiating weapon is plan B.
BATNA stands for “Best Alternative To a Negotiated Agreement” – your best option if the negotiation fails. Fisher and Ury coined the term in 1981 in “Getting to Yes”. With a competing offer in your pocket you negotiate calmly and reject bad deals. Without an alternative you accept almost anything. No deal should ever be worse than your BATNA.
Useful: Before any negotiation, write down what you will do if no deal is reached.
Source: en.wikipedia.org