Everyone does this differently. Me, I take a lot of notes when I first go through the books, add sticky tabs to anything that catches my eye, and seperate entries into piles. After all that's done I print out the list of every product that has been entered uring the year, and go through them all, entry by entry, highlighting the list so I know that I've checked everything.
As I look at each entry I decide which categories I might be interested in nominationg it for. There's no real attempt to narrow down the lists at this point. I just organize it a bit, and add anything I think might have a real chance for my nomination. Each category gets a piece of paper, anything that might fit gets it's name added to the list.
It's during this stage that I go back and re-read anything that I don't comfortably remember, and make an effort to go through every product page by page. Other than initial reading this is the most time-consuming part of the process, and it's what I'm working on right now.
After this I'll go through the lists for each category and narrow them down to five products in each. I have a couple more weeks before I have to be done with this part, but when I do finish it'll be time to compare my lists to the other judges'.