Something that has come up repeatedly in the past year is that quitting things entirely is often easier than reducing.
Some recent occurences were a Tim Ferris Q&A where he talks about the 'one decision to remove 1000 decisions', and in Clay Christenssen speech to HBS graduates where he doesn't play ball on Sundays. "Just this once" means the question can come up literally every time the possibility exists. And you can waste a lot of time thinking up ways to justify it that still fit into whatever rules you have.
Last week I tried to reduce coffee drinking to one per day. One both Monday and Wednesday I ended up having three! This is partly out of habit, partly out of a cheat-day mentality, Easier is to stop entirely; I usually do this when I travel.