I use the back of paid bills, opened envelopes, receipts etc to jot down notes, phone numbers, names, dates, or todos whenever I need it (usually during a phone call or while "cleaning" or reading emails on the bus).
I keep them visible on the table or in my bag until they have been completed and then thrown them away. Or until I lose them, and if nothing bad happens it probably wasn't that important to keep anyway. I try to transfer important notes to more durable versions, or gather todo-scraps or half-done lists into one clean list on a bigger scrap frequently.
I tried keeping notes on my phone or in a notebook, but it's never there when I need it plus I get overwhelmed with decisions regarding organising when so many different notes need to be gathered in just one place (how do I separate work from private from volunteer-notes? What do I do with things that will be outdated in a week? Do "buy soil or make own?" really belong in the same book as "breathe, you fool!" and "monday: bus 7:37, pack bag and pee 7:20"? And how do I find the notes I need to read often among the notes I only need to real when they are relevant?)... so I just never get a good system going.