[anonymouz]

The automatically-lock-when-shut behavior of doors is an extremely weak protection anyway, so one should always lock the door with a key anyway (this at least prevents the most trivial circumvention, i.e., open the door with a plastic card).

Interestingly I seem to use more or less the converse of your method, also with great success: Always lock the apartment door from the inside. Whenever I leave the apartment, I therefore am forced to take the keys.

I suppose the most important thing is to be consistent with ones behavior, so that a subconscious habit can develop.