In his sermon on Luke 18:9-14, the parable of the Pharisee and the tax collector, Brunner writes
Does this imply that we are continually to come before God with this prayer on our lips: "God , be merciful to me a sinner"? Yes , this is implied. Even more. Only in this humility do we actually stand before God in prayer. You see to stand before God is a great mystery. It is not enough to say the words of prayer. We stand before God at the very moment when the thought strikes us: I am a sinner; God have mercy on me! And as soon as this thought, this insight, vanishes from our hearts, do we vanish from the presence of God, even though we may go on praying for hours.
Emil Brunner, Sowing and Reaping, Epworth 1964, p23
To which I'd add that the Lord's prayer commits us to our position as sinners before God every time we pray "forgive us our sins."