I keep a pocket size notebook for each of my snakes. I record feeding dates, whether they ate or refused, type of prey (rat, mouse, prey weight, ft, live, etc). I also record when they shed, and when they poop. Usually the same day they poop, I get an empty weight and log that as well. I like to see a broken down progression of their growth.
Other than that, I put any random notes I deem necessary like anything out of the ordinary or concerning in case they need a vet visit.
I think when I start breeding, I'll print up log sheets for babies with their picture and information then keep notes in a binder. That way I have something less bulky but equally informative to give to anyone I sell a hatchling to.