Practice negotiating on relationship topics. If every negotiation turns into an all-night crying jag or blamefest, then negotiation will not happen because you will be afraid of it. Practice various sharing and listening exercises until you can negotiate effectively even on emotional subjects.
Bring up hard subjects. It is unfortunately common for people in relationships to avoid hard subjects for months or years, making things much worse when they do come to a head. While there are more and less diplomatic times to bring something up, do not use the “it’s not a good time” excuse to put something off indefinitely. At the same time, do not browbeat your partner by repeatedly bringing up the same subject.
Do not negotiate while upset or freaked out. Take a break (possibly of a couple days) or a long walk if you need to. If the negotiation is happening because of a surprise jealousy experience or similar surprise, wait a while before addressing it.
Do not chase your partner(s). If they say that they cannot discuss this right now, they probably mean it, and any amount of chasing them around the house will not change that (and can hurt your relationship). Give it a break and try again later. However, do not allow this to become an excuse for avoiding difficult conversations.
Create space in your negotiation for “illogical” emotions (which are rarely as illogical as they might seem at first). Admit the emotions you are feeling. Practice listening without judging or interrupting. Acknowledge your partner’s emotions without feeling like you need to necessarily do something about them, or that you need to fight them. Do not try to make all your arguments logical, and do not try to use logical arguments to conceal your emotions on a subject.
Dig into the reasons behind your negotiating position. If you can establish a chain of reasons for the way you feel, often this will provide creative solutions that are satisfactory at a different level than the initial concern that started the negotiation. Ask your partner(s) to describe into their reasons, and then listen to those reasons non-judgmentally. Do not try to devalue your partner(s) arguments by trying to show they are baseless.
Bring a can-do attitude to negotiations. What can you do to make your partner feel safer, more free, or more loved? What compromises can you make? What relatively small things can you compromise on in order to get the big things you want? Even if you cannot get the big things, what would you be happy with to start? Always bring some level of compromise to the table: sticking to a hard line will only ensure that you remain at an impasse. This does not mean you should let someone else walk all over you.
Be willing to not solve the problem in this round of negotiations. Bridging hard differences will take a number of periodic negotiations, sometimes over the course of months or years. Also, try to keep the negotiation short.
Negotiate for the short term. People tend to assume that whatever they are figuring out will apply to the rest of the relationship, and this tends to put people at an impasse. Make agreements that only last for a certain amount of time: a week, a month, two months. Negotiate for certain events, like a weekend away. Make it clear that what is determined only applies within the time limit, and you will need to renegotiate when the time limit is up.
Leave space for reopening negotiations. Leaving a space open for renegotiation prevents people from feeling trapped, which helps them honor any agreements. Creating a hard permanent rule just encourages your partner(s) to break it. Of course, this should not be abused: do not try to open a renegotiation just to take advantage of a particular situation.
Do not allow D/S dynamics to derail your negotiation. If it is not possible to take a short negotiating hiatus from the D/S dynamic (which is the case in many D/S relationships), then somehow work it into your D/S practice. For example, by having a mechanism where the submissive can make nonmonogamy requests of the dominant, which are judged on a case-by-case basis.
Negotiate even when there is no obvious need. Not only does this keep you in practice, but it gets you to brainstorm possible scenarios, which can be the key to avoiding nasty surprises later.
People will always hear different things. We seem to always interpret agreements in our own personal best interest. Be specific when negotiating. If you find that one or both (or all) negotiators have divergent interpretations after the fact, take to writing things down. Do not be surprised or derailed when this sort of divergence happens, as it almost certainly will.