I am posting anomously because my parent company has a highly ritualized performance review process and may not take kindly to criticism of it.
The best way not to stare at a blank piece of paper for hours is to realize that the excercise has no value and just get it done. List 4-6 items that one has accomplished over the year and skip any analysis. What counts is not how well or poorly one thinks a task went, it is how the boss believes the task went.
The evaluation rarely has anything to do with one's personal effort or ability and more with the final result. It is better to do poorly on a successful effort than to perform exceedingly well on an effort that ultimately fails. In 30+ years of ging through this ritual, I have never received any benefit from this process. I have typically been ranked at or near the top in these evaluations, so I am not bitter. In all that time however, I have never received any indication of what I have actually done well or what improvements I should make.
From the management side, if a report is doing poorly, I will address it at that time and not wait for the review process. By that time, we will have either made improvements or removed the report. If someone has done well or worked extremely hard on something, I will also make sure to publicly thank him and forward that to his supervisor or manager.
By the end of the year, management's views have been already solidfied and additional facts or spin are not going to shift their views. Just list out what one has done, don't take the positives too highly, and don't take the negatives too seriously. As long as one has done his best, that is all that can be asked.