Since most people find the unknown a bit scary you'll want to ask yourself what approach to the negotiating table you find most comfortable. In short, plan your strategy. Be sure you know exactly what you want, not approximately. This does not mean you will ever get exactly that, but having information clear in your head will help you determine what you are willing to concede. Unless you know what you want, you won't be able to tell somebody else.
Clarity improves communication, which is the conduit for effective negotiations.
Practice Actually rehearse the presentation in advance using another person as the employer. If you make mistakes in rehearsal, chances are that you will not repeat them during the actual negotiations. A friend can critique your reasoning and help you prepare for questions. If this all seems like a lot of work, remember that if something is worth negotiating for, it is worth preparing for. Be prepared to support your points of disagreement, outlining the parts you would like to alter, your suggestions on how this can be done and why it would serve the company's best interests to accommodate your request.