Can you share a little more context? Does this determine your eligibility for a promotion or raise? Is your job on the line? How many other people will you be compared to? What kinds of categories are you asked to give numbers to?

I would recommend a good brainstorming session on the topic “significant things I did this year”. Just start writing down everything memorable or out of the ordinary, and see what falls out.

For numbers, I would ask the following questions: “Could I have reasonably performed better in this area than I did?” and “How did my peers perform in this area?”

If there’s something that went wrong that was in any way not your fault (e.g., missed deadlines due to unclear objectives from management) this is likely a good place to make mention of it.

Don’t negotiate against your self.

Go big on the numbers; good people tend to be too self critical.

Excellent advice above on making sure to list your accomplishments.

There’s a lot of pressure on managers to keep your rating down, so you need to give him or her the ammunition to prove to HR that you really are outstanding.