Memory for now, but I do keep an excel (which probably hasn't been updated) of G1 figures I need through 88, and have moved to having pictures of checklists where I cut them out as I obtain them. However I do need to do a better job because some of mine are incomplete, and my memory isn't good enough to keep track of what parts I need from back then. Jetfire is one that I really need a parts list for as I have one from my childhood that is missing or had parts break, as well as a MIB one that is missing a couple pieces, but I have no idea which ones.
It's all rote memory for me. Since I don't typically buy multiples of anything, I can remember what I have quite easily. Preparing and maintaining a spreadsheet is too much maintenance... and makes it feel more like work or a chore than a fun hobby.
