Bipolar disorder can be a difficult illness to treat, because the symptoms vary from person to person. It's also possible for the same person to experience bipolar episodes differently from one time to the next (for example, one manic episode might involve feelings of euphoria, but the next episode feels more angry). If you already think you have bipolar disorder, this puts you ahead of the game, so you can go to your appointment armed and ready!
First, if you haven't already, complete the Mood Disorder Questionnaire (this is available at www.dbtforbipolar.com). Print out your results and bring this to your appointment. Second, you may want to start working on a Life Chart, which is a time-line on which you would chart as many depressive and hypomanic episodes as you can recall (for more information on life-charting, see The Dialectical Behavior Therapy Skills Workbook for Bipolar Disorder). By thinking back and making notes about previous "highs" and "lows" you've had, you'll be able to provide your doctor with a lot of really helpful information.
Last, and maybe most important, if you have a family member or other loved one who can attend the appointment with you, information from someone else is always helpful in diagnosing bipolar disorder. While you obviously feel what it's like when you're experiencing an episode, a loved one can report on what changes they see in your personality and behaviors, and so on. The more information you have, the more likely you'll be to get an accurate diagnosis.