Negotiate on Purpose

As simple as this sounds... begin with the end in mind. Know specifically what it is that you are trying to get out of your negotiations.

When you know your desired outcome everything can become a path or tool to help you reach your target.

I agree with this 1000%. If you don't know what you're after how will you know when you've got it?

I think it's not uncommon at all for parties to enter into negotiations without having clear idea of what they want and what they will or will not do to reach their desired outcome.

IMO, it is good to enter into a negotiation with both a "best case" result and the minimum result that is acceptable in mind. Always aim for the "Best" result, but know what else you are willing to accept.