Don't screw yourself from the get-go. Many company request your salary requirements with your resume. Do not include them. Very few companies will hold it against you, and this information is only to their advantage. If your number is too small, there is no room to negotiate, and if it's too high, you won't get a call back.
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.