A map, which I print out and, as details appear, I use post-it notes to add these to the map.

As a pantser, it just means I don't forget something. But for a plotter, maybe the physical (as opposed to electronic) map is something that can help. So, the person doing NaNo with me is a bit of a plotter, and her story is set in the town where we live (it's a romance). She's printed up the map and it is about 2m by 3m and stuck on the wall of her study. She has put people and shops and stuff all over the place as they come up in her planning, plus she has named and indicated real-world shops, etc. that she needs for the tale.

As an aside, she has taken the time to go into the places she will include and ask if they are okay with her mentioning / utilising them. So far, none have said, "No."