Make a list of things that you feel are very important to you and also create a list of things that are not very important to you. And if you know, then also make a list of things that are important to stbx.
For the things that are important to you, rank them. Also tried to get as detailed as possible about what about those things is most important. That will help you say "No, what your propose is not acceptable".
For the things that are not very important to you, see if there is an overlap of things that you think are important to stbx. What does HE want most. During the negotiations, you can take a stand that you must have that one thing he wants, and then trade that for something that you want.
IN summary, go into this knowing what you MUST have, what you want, and what you don;t really care about. And then fight for what you MUST have.