The most important thing to consider when you go into a an evaluation is to come prepared. Not only should you bring a copy of your self-evaluation, but you should also bring any data or facts that support the comments made in your evaluation. If you claimed that you saved the company x amount of dollars, then have something to prove that you did.
Make sure to keep your voice and emotions in-check when discussing your self-evaluation. If your supervisor approaches the session by just giving general comments, ask him or her to give specifics. If your supervisor approaches the meeting with generalities, ask to go over each item in the evaluation point-by-point. If you disagree, calmly bring up your counter points and support. If your supervisor has made a written document, it is entirely within your right to ask for some time to review the comments before you have the discussion--a good boss should have this built into the process.
At the end of the session, if you are still not happy, you can refuse to sign the document. You are usually given the opportunity to write a rebuttal or to ask for a meeting with your supervisor and HR. In this day and age, it is important to realize that companies manage evaluations (those that impact salary) according to a fixed monetary amount. In some companies, your rating will be downgraded simply because the company cannot afford to pay you for a higher rate. For instance, if your department has a flat budget and someone was promoted, it is likely that others will have lower raises.