For just these reasons--as a basis for letters and for performance reviews--I do what Nanci does. Every year, I open a document called Goals and Achievements and I list every little thing I do at work in that document, from the mundane to the unusual. It's also a good place to keep track of problems and what causes them--lack of resources, etc.

I can't remember a job in which I didn't have to provide at least some input for my own performance review. And it kind of makes sense--you're more likely to know than your boss is exactly what you've done and how well you've done it. If you can't blow your own horn, you can't expect someone else to do it for you.