Self-assessment is particularly effective because you can do it between official job reviews. This allows you to make corrections without management's prompting. If you have done your groundwork, and you correctly view your job as a partnership between you and management, you will have a good idea of what your boss is going to review you on.
If you are honest, you probably know what your boss will focus on during his assessment. A few times when counseling technologists on their careers, I have heard about a poor performance review, with the employee complaining that he did not know what his boss was looking for.
I have to tell it to you straight. For all the times I've heard this, I am convinced that it is rarely the case. What makes a good worker is not mysterious:
Outside of these key ideas, little elaboration is required.