The single biggest source of strength in a negotiation is not charm or toughness. It is your best alternative if this deal does not happen — what negotiators call your BATNA, the best alternative to a negotiated agreement.
If you are buying software and another supplier has already quoted a workable price, you can walk away from a bad offer calmly. If you have no other option and a deadline next week, you will feel pressure to accept almost anything. The other side has an alternative too, and estimating it is part of preparation.
Your alternative also sets your walk-away point: the least favorable terms you would still accept. Anything worse than your alternative should be declined. Writing this number down before the conversation protects you from conceding step by step in the room.
Finally, you can improve your alternative before negotiating: get a second quote, line up another candidate, or prepare to do the work in-house.
Try this: pick an upcoming negotiation. Write your best alternative, your walk-away point and one action that would improve your alternative this week.