I keep track of feeding, shedding, poop, and weight. Usually the latter two at the same time since I like to weight them empty. It gives me a better idea of how much they're growing. Or not growing. I keep track of breeding too, but I have a whole nother spreadsheet for that. And it's all on the computer. I don't have time to deal with cards or paper.

I guess that might be extreme, but I'm a stickler for details and I have a small collection. If I had 50+ animals I'd probably have to streamline things.