I'm not nearly as organized as I want to be. I usually do manage to take pictures of finished objects before giving them away, though, and to jot down a few things.
What I do reliably keep track of, is the yarn and needle size used for the pattern, and the gauge I got. I keep a list of this in the back of my "Handy Book of Patterns," which is usually in my knitting bag. When I use the same yarn again, I don't have to knit a swatch for it, because I've noted what gauge I got with the needles! This saves so much time! It's also handy to be able to refer to the gauges of yarns I might have extra of, to see if they'd be compatible with a new project.
That book, BYW, is a great carry-with-you book. It has pages for notes, and a huge pocket for storing folded patterns and paper. It also has a small but usefull illustrated glossary of knitting basics. It's great for jotting down essentials you want to have with you in your bag, and for toting graph paper and patterns, etc.