What the Keeping of the Heart Presupposes and Signifies

(NB: all italics original)

Naturally, to keep the heart presupposes regeneration. For without it, there will neither be the ability or desire to keep it. Unless grace has made the heart right, it cannot be maintained.

Man, by the apostacy, [sic] is become a most disordered and rebellious creature, opposing his Maker, as the First Cause, by self-dependence; as the Chief Good, by self-love; as the Highest Lord, by self-will; and as the Last End, by self-seeking.

This soul-disorder is reset through regeneration, "the renovation of the soul after the image of God."

self-dependence is removed by faith; self-love, by subjection and obedience to the will of God; and self-seeking by self-denial. The darkened understanding is illuminated, the refractory will sweetly subdued, the rebellious appetite gradually conquered. Thus the soul which sin had universally depraved, is by grace restored.

That's basic. Every Christian knows that to some extent--maybe not phrased as nicely as Flavel gets it, but the meaning is known. Given that, it's pretty easy to figure out what he means by keeping the heart.

nothing but the constant care and diligence of such a renewed man to preserve his soul in that holy frame to which grace has raised it.

Even tho' the soul has been rectified, been restored, given "an habitual heavenly temper," sin often disturbs it...gets it a little off course, and it needs to be set right again.

To keep the heart then, is carefully to preserve it from sin, which disorders it; and maintain that spiritual frame which fits it for a life of communion with God.

Flavel lists six parts of this, some of which seem self-evident...others, not so much:

1. "Frequent observation of the frame of the heart." Not a morbid introspection, but an honest appraisal of the state of your heart. "The heart can never be kept until its case be examined and understood."
2. "Deep humiliation for heart evils and disorders." Sin affects our hearts. Period. We have to it for what it is--disgusting, disturbing, something to be mourned over and ashamed of before we can move on.

   if a small dust get into the eye if will never cease twinkling and watering till it has wept it out: so the upright heart cannot be at rest till it has wept out its troubles and poured out its complaints before the Lord.

3. "Earnest supplication and instant prayer for purifying and rectifying grace when sin had defiled and disordered the heart." Repentance is essential. It comes, as the catechism says, out of a true sense of one's sin, and apprehension of the mercy of God in Christ. Flavel prays as an example to his reader,

   'O for a better heart! Oh for a heart to love God more; to hate sin more; to walk more evenly with God. Lord! deny not to me such a heart, whatever thou deny me: give me a heart to fear thee, to love and delight in thee, if I beg my bread in desolate places.'

   May I learn to pray like that.
4. "Imposing of strong engagements upon ourselves to walk more carefully with God, and avoid the occasions whereby the heart may be induced to sin." It is frequently helpful to be like Job who made a covenant with his eyes. Maybe you have a different organ of 5 to make a covenant with--maybe just your mind, but to guard against some special sin, such a covenant can be "very useful." "By this means holy men have overawed their soul, and preserved themselves from defilement."
5. "A Constant and holy jealousy over our own hearts." We must be on constant watch for the stirrings of affections, the beginnings of temptation, and react against it. Be on guard from the stirrings.

   Happy is the man that thus feareth always. By this fear of the Lord it is that men depart form evil, shake off sloth and preserve themselves from iniquity.

6. "The realizing of God's presence with us, and setting the Lord always before us." When we remember we are living coram Deo (thanks, Dr. Sproul), when we meditate upon what that means--"we dare not let out our thoughts and affections to vanity."

These are the ways in which Christians express the care they have of their hearts. Preventing corruptions from erupting in times of temptation, careful to keep the sweetness and comfort received in religious duties.

This is the work, and of all works in religion it is the most difficult, constant, and important work.

this has nothing to do with anything, but isn't the phrase "refractory will" a keeper?