I started keeping a log book at the pistol range some time ago. A range book makes a lot of sense on the rifle range, but apparently is not really common practice on the pistol range. It's useful for a few reasons, however. I noticed that on some days I was just loading and shooting, and not really following proper technique. Whether it's because I buy ammo in bulk and I stop paying attention to every round (rifle ammo is expensive compared to pistol ammo) or just lack of discipline, it started to worry me. Someone posted on these forums a few months ago that "practice doesn't make perfect. Perfect practice makes perfect." Sage advice.
Maintaining a log book helped a lot with perfect practice. For one, it forces me to step away from the lane (in a fugurative sense) and really think about what I was doing. For another, it makes it easier to see trends across days and weeks. For example, when I shoot certain types of .40 caliber ammo, I tend to pull my shots high. Now there's a lot of human variation on a day to day basis that cannot be pinned down, but general trends are easier to spot. From a maintenance standpoint, it's also useful. I know how many rounds have gone through each barrel. I know what ammo works with what gun.
Anyhoo, some of the data I capture:
Date, time of day (roughly), Gun, Distance, Ammo, Lane #, number of shots, FTF/FTE, misfeeds and of course shot placement.