There is a saying “failing to plan is planning to fail”. That is exceptionally true when it comes to negotiations. Planning will ensure you're conscious of your objectives and requirements. Although you will need to be a quick thinker and be flexible, in case the other party makes a decision you didn't plan on, you can't just be spontaneous. You have to realize that both parties are interested in mutual problem solving. Remember that you're entering into a relationship therefore you need to be aware of the long-term implications of any decision you make.