For the boss - Take notes throughout the year on the good and the bad. Put them in a notebook that's not used for anything else, on a thumb drive, whatever works for you that is always handy. Set aside an hour every Friday (or Monday if that's quieter for you) to record your observations from the previous week. Don't make your notes cryptic - you'll be trying to make sense out of them several months from now and may not remember what "gd prstn - DF/JA diff" means. If you have them electronically, it's easier to go back at the end of the year and group things based on performance plan elements.
It's just as important to take notes on the good stuff as the bad and the so-so. You're probably going to have more conversations about the bad during the year to fix performance problems and you'll remember those. But that good job on a project in the first month or two of the rating period may slip your mind.