Compare your interests to the person you’re negotiating with

Ask the other person what their interests in the negotiation are
Listen carefully and note them down on a sheet of paper - even the ones that conflict with your interests. If new ones occur to you throughout the process, make sure you add them as well.

Ask them why they care about these interests
Try to dig deeper to really understand their motivation. Rank them according to your estimates for order of importance.

Look for shared and complementary interests to seal the deal
Next to their interests written down, add yours. Use these notes during negotiation.