An appraisal shouldn't be a situation whereby you have to justify your position with the company, nor should it be an opportunity for your manager to tell you all your faults. I used to think it was because this was always how mine turned out.
When I started working for my current employer, I was pleasantly surprised. I was given a crib sheet beforehand outlining what would be discussed - performance, achievements, areas for improvement, training needs etc etc. and my subsequent appraisals have always been preceded by my being given a copy of the notes from the previous one. I've also been pleasantly surprised when my boss has told me what she thinks I've done well, as well as where she feels I need to improve (with the result that we usually tend to agree on the latter).
So yes, afcjan, emphasise what you feel you've done well, but keep in mind what's not gone so well and be prepared to discuss that. It could simply be a matter of training. My biggest bugbear is that of time management, for instance. I'm not good at it. But we discuss this in appraisals and because my boss is aware of it she is tremendously supportive in my efforts to get it under control. Means we can work on the problem together without there being any worker/manager resentment. To me, that's the whole point of having regular appraisals.