First, do not be a passive participant, just showing up and listening. At a former firm, we associates could fill out a sort of self-evaluation report, where we could describe significant projects/achievements, as well as professional development activities, pro bono, firm activities (eg., recruiting committee) and community involvement. Although this was voluntary, and it was time-consuming to do what I perceived to be a thorough job, I forced myself to do it, figuring that the primary evaluator (or any evaluators who received it) probably didn't know everything I had done all year, and this would help them understand the big picture. Remember, the senior attorneys who complete written evaluations have several to do and it is an arduous task. They appreciated that I gave them information that they could plug into their evaluations. And, I felt better knowing that they were more aware of all I was doing. If you are not aware of such a form at your firm, ask around. I gave a junior associate a copy of one of my older self-evaluations that she could use as a model.
If there are particular issues you would like to discuss (say, I am a junior litigator, but I haven't defended a deposition), you may wish to drop the primary evaluator a note with some things you would like to discuss. Supervising attorneys generally like it when we see someone who is interested in furthering his or her professional development.
Have a sense of your numbers before you go in (e.g., hours, client origination (if applicable), etc. This helps if you get your evaluation and there's incorrect numerical information or numerical-based observations.
If you get a copy of the written evaluation before, review it carefully. Make notes of any incorrect statements or any items with which you disagree. You can be prepared to discuss -- calmly.
Bring a notepad and pen, you may wish to take notes.
Listen to the evaluator. Be appreciative of compliments. Take in constructive criticism. If there is something you do not understand, ask calmly for clarification. Oftentimes the evaluator has collected various comments from other reviewers, and may need to go back to any underlying evaluation form to determine what is meant.
If you have information to share -- like hours information is incorrect, or you actually did defend a deposition when they said you still needed to do so -- share that with the reviewer. While you shouldn't be overly defensive or abrasive, this is YOUR career and you shouldn't sit passively like a pillow on a couch. I often was able to get things added to my evaluation that were overlooked.
Feel free to express appreciation to the evaluator for his or her efforts. These evals do take up a lot of non-billable time.
If there are opportunities you would like to get, especially with the evaluator (e.g., work on the xxx case, or a xyz type of matter), go ahead and express that as well. This is usually a good time to indicate the areas you are interested in, and to show that you are more than an employee -- that you want to develop your career and expand your capabilities and experiences.
No crying. Doesn't help.
No whining. Only hurts.
Be professional, be calm.
-- These words of advice assume that you are not getting canned in the evaluation or otherwise "set up." We can address that situation another time.