Negotiations are everywhere - at war, in trade, in personal relationship, even in relationships with yourself. Hereby I list some hints as they come to mind either internally or from external sources.
Negotiations process is a poker game. So same hints apply. Another science is mathematical game theory, however this one is way more sophisticated.
Don't trust. War is an art of lies - Sun Tzu.
Less talking, more questioning. Ask as many questions as you can, get as much information as you can.
Don't insult. Threat can also be an insult so use it only as a last remedy. If the other side isn't afraid to lose it can fight till the end just because it is insulted.
Less proposals, more inquiries. Let the other side make a proposal first so you know their limits and you have more information. Make your own proposal only in the end.
Plan B. Always have plan B for any step. Especially for such dangerous steps as proposals and threats. What if you proposal is rejected, what if they don't care about your threat.
Break down demands into small parts. If there are multiple demands, one of them is always major. So we need to find which one.
Don't be afraid. Turn off your fear. Fear prevents us from making good decisions. Use logic instead.
Don't take a decision under pressure. The other party might make a rush. We need to distinct serious ultimatum and alleged rush.