To guard the mind requires that we know Christ's presence in our heart. Once we know this presence, we can bring our innocent thoughts to Christ. But first, our heart needs to be opened. Until it is open, Fr. Dimitru Staniloae says,
we must knock at its door, with thoughts sacrificed to Christ, with the hope that we will gain the awareness of His presence and by this our heart will be opened.He also says that we don't have a full feeling of His presence at first. We will experience gradual progress in this. We must be persistent and have patience.
The whole notion about guarding the mind is dependent on us being able to bring our thoughts to the door of the heart. Therefore it is also called watching of the heart.
Fr Dimitru says,
Standing watch at the door of the heart, the mind does nothing but keep itself from going astray, because the heart is after all nothing but the depths of the mind.Think about how often our minds go astray. How often by our immersion in our ego needs we ignore this place of the heart. The mind never stops and our actions seemingly spinout of the control of our highest values. We need to be ever vigilant.
Mark the Ascetic says,
The mind must keep vigil over the heart and guard it with all watchfulness, trying to penetrate into its innermost and undisturbed chamber, where there are no winds of evil thoughts... to be vigilant over the heart and go ever deeper into it and to approach God alone, with out becoming disgusted with the toils of attention and persistence.We have to train the mind to be the supervisor and to watch the thoughts as they enter. At the same time to be aware of the presence of Christ within us, in the heart. Then our thoughts can be presented to Christ as a discipline.
Fr Dimitru says this is how it works,
First a simple thought appears in the consciousness. Immediately somewhere on the periphery an evil thought shows itself, with the tendency of monopolizing the simple thought.But often this watchful defense fails. We allow our thought to become associated with to a desire.
Fr. Staniloae says,
The mind forgets itself for a little and lets itself be touched by the gentle breeze which is coming from the aroused appetite; it finds that it was robbed of its simple first-born thought and was bitten by the passions.Even when we let our guard down allowing a thought that arouses our passions, our approach is to still refer this thought to God in our heart. We must now call on Him with all our power––"Lord have mercy, Lord have mercy, Lord have mercy."
We may also experience the first thought as a direct attack and not an innocent thought. We need to develop that capacity to recognize it as a direct attack at the very beginning.
For Dimitru says,
It must however, be unmasked at the start, so that we will scarcely be able to escape.He then tells us that this calls for a special spiritual sensitivity which is gained only through steady practice and much effort to cleanse ourselves from the passions.
The challenge is developing continual prayer so we will continually have God on our mind. We need to be able to at any time stop and say, Lord have mercy, or to recite the Lord's Prayer, Our Father...
Dr Dimitru concludes this topic as follows.
Guarding thought... consists of a continual reciting of the name of God in the mind, in the seeking of the heart, or in concentrating within it. But nothing but a concentrated uninterrupted prayer...This is the meaning of Saint Paul's teaching on unceasing prayer.
pray without ceasing...test all things; hold fast what is good. Abstain from every form of evil. (1Thess 5:17, 21,22) Rejoicing in hope, patient in tribulation, continuing steadfastly in prayer... (Romans 12: 9,12)This seems to be be an advanced spiritual practice demanding lots of self-control, a life of repentance, and significant progress in mastering the passions.
Reference: "Orthodox Spirituality" By Fr.Dimitru Staniloae