The person with fewer acceptable alternatives often carries less leverage.

Leverage is mostly arithmetic about options. Whoever can say no and still be fine is negotiating from a different place than whoever needs this particular agreement to work, and no amount of preparation or confidence changes that underlying count. It is worth knowing which of you is in which position, honestly, before you begin. Pretending otherwise tends to be expensive.

Count your real alternatives before the conversation and try to add one. Even a mediocre second option changes what you can hold. If you truly have none, negotiate for the relationship rather than the terms.