Schedule a Future Interaction Before a Negotiation

Your counterpart will negotiate less aggressively if you schedule a future time to meet.

Calendar with "Negotiation", followed by "Review Contract"

Overview

People feel obligated to act favorably if they will be interacting with somebody again:

When social dilemmas involve repeated interaction over a period of time, people often develop a readiness for mutual cooperation... [This] implies that the only way to succeed is to get the other(s) to cooperate. (Pruitt, 1998, p. 474)

Even if you only meet once to negotiate, remind them of a future interaction:

Finalize Agreement. You'll reconnect to review the contract (Murninghan & Roth, 1983).
Conferences. You'll be seeing each other at industry events.
Future Projects. You'll work with each other on new projects later.

Murnighan, J. K., & Roth, A. E. (1983). Expecting continued play in prisoner’s dilemma games: A test of several models. Journal of conflict resolution, 27(2), 279-300.
Pruitt, D. G. (1998). Social conflict.