Thus consider that you are always on duty…

…  required to firmly apply your full attention. If you slack off, you’re useless.

Remember such a stable and firm attitude is itself meditation practice. There is no other method of concentration to seek. Buddhism itself is about applying full attention steadily, without being disturbed by external things. Developing a confident attitude that is never pained or vexed or worried or saddened or altered or frightened is called attaining Buddhahood.