Preparation is probably the single most important part of successful negotiations. Any good trial attorney will tell you the key to presenting a good case in the courtroom is the hours of preparation that happen beforehand. The motivation is easy. Granted, it's a complete pain in the ass, but every job seeker should put together a rough personal budget, keeping in mind that things like income tax, loan payments and health care will probably account for a huge chunk of each paycheck. Be sure you know exactly what you want, not approximately.
Clarity improves communication, which is the conduit for effective negotiations.
Justify your needs to yourself, and you will be confident enough to get it. Gather as much factual information as you can to back up the case you want to make. For example, if most entering employees cannot negotiate salary you may be jeopardizing the offer by focusing on that aspect of the package. Turn your attention to some of the other more flexible parts of the offer in order to sweeten the deal, such as your health plan, dental plan, retirement package, the type of schedule you prefer, etc. Review what you know about your contact's communication style and decision-making behavior.