Since most people find the unknown a bit scary you will 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 will not be able to tell someone 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 seems like quite a bit 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.