I keep an Excel spreadsheet with charts on each animal which I regularly print out to keep a hard copy in case of data loss. The copies go into a binder with blue tabs for the females and white for the males.
I track feeding, what they ate, when they defecate (short tails only), in shed, the day they shed, copulation, weights on a somewhat regular basis and when they refuse to feed. I feel like I'm forgetting something too.
All of them gets an ID number, twelve characters in four groups of three which denotes the collection number, species, when I got it, if it was born here or acquired, and male or female.