I keep all my notes in a leather-bound blank book. I write down everything about a brew starting as soon as I choose I style. I write about why I want to brew that style, what characteristics I want to nail, commercial examples I want to emulate, etc. I then come up with several rough recipes while researching in books and on these forums.
On the brew day, I write down the exact grain bill, mash temps and times, weather conditions (my mash loses much more heat on a cold windy day), details on the sparge, hop schedule and alpha acid of all hops used, original gravity, yeast variety, pitching rate and temperature.
I take notes on fermentation temperatures and times. I record the final volume of the batch, how much and what type of priming sugar I use, and how many bottles it fills.
Once a beer has had adequate time to bottle condition, I write up several tastings over the lifetime of the beer. I find this is really helpful for duplicating great beers, or understanding how changes in recipe and technique change a beer.
Although it makes for an interesting cover-cover read, my chronological method makes it kind of annoying to track just one beer. If you made all your notes on loose-leaf, you could toss them in a binder and arrange all the notes for each brew grouped together.