For what it's worth, about four months ago I gave myself a rule that felt too dumb to type out: anything I eat gets eaten sitting down, at a table, off a plate or out of a bowl. No grazing while I load the dishwasher, no finishing what's left on someone else's plate, no handful of nuts over the sink.
I logged the slips in the same notes file I use for everything else. The first two weeks, five to seven a day. Last month, twice total. Cumulative, somewhere around 40 slips since I started, which sounds bad until I compare it to the hundreds of unlogged bites I'd have taken otherwise.
I'm not claiming this is some mechanism. My honest guess is it just makes portions visible and buys a five-second pause where the "do I actually want this" question has room to land. The prescription helps too and I don't want to credit a habit with work it isn't doing.
So what's your smallest rule — the one that sounds too obvious to say out loud but you still follow it?