I carry a Moleskine notebook that I keep notes in. When I photograph, I write the date, location, and name of the place, person, or thing I photograph. I also note what kind of film I used and which of my cameras. This all helps me because my work is documentary in nature. A lot of my early work was done before I kept notes, and now I cannot remember when I made the photos, and in some cases I cannot even remember WHERE I made them. Very frustrating! It doesn't take long to note this stuff after I finish photographing, and the notes have been invaluable to me later on. I have a big shelf full of these notebooks now, since I have been keeping notes like this for 8 years.
When I develop the film, I keep notes in a separate set of notebooks with the info on how I developed each roll (developer, dilution, temp, time, agitation). Each roll is numbered, with the roll number in the notebook and one the plastic binder page each roll of film is stored in. I also write the date and subjects in this notebook. These Developing notes have been helpful in letting me keep track of what does and does not work.