Bullet Journal — the original method described by Carrol, not the fancy spread-and-decoration bullshit. Write everything down as it occurs to you, indicate whether it's a note or a to-do with different bullet points, and at the end of the day decide which tasks to put in tomorrow's notes and which to discard. Date each page and list the page number in the index of it contains long-term notes (eg. quite from a contractor, birthday present ideas).
It's easy, it's fast, and it doesn't break if you forget to do it for a day or two or two hundred.