[ethanbond]

Well this is kind of "smuggled in." If you are flossing one tooth, you need to have floss readily available. The point is that you are eating the (yes, usually significant) startup cost and that's it. Keep doing that until it's not even a cost for you, then you can start eating into the variable cost of a more complete habit.

As far as knowing to initiate the habit, place it directly behind an already established habit. Flossing is easy since you probably brush your teeth regularly.

[alxlu]

I think what I'm trying to say though is that there's no benefit to starting with one tooth if you have already paid that "startup cost". Since at that point the cognitive load of flossing one tooth vs flossing all your teeth is basically nonexistent.

[ethanbond]

One tooth is strictly less time, less commitment, less cognitive overhead than all your teeth.

Simple as that.

Of course this is just the regime that helped me build 4 or 5 brand new daily habits in under two years, all of which I had tried and failed several times before. YMMV.