We guard most closely what we most treasure.
Solomon, out of his painful experience, pleaded with his children,
Above all else, guard your heart,for it is the wellspring of life.(Proverbs 4:23)
What is it you most protect? Your reputation? Your investments? Your family? Of all you guard, says Solomon, guard nothing with the care and strength you guard your affections. Once your heart latches onto something, you will not be able to stop your will from consenting to it.
...Fix your affections on God himself, in his beauty and glory. Fix your heart on the Lord Christ, the fairest of ten thousand, the desired of the nations. Get worked up about the mystery of the gospel, all the wisdom and love of God displayed in Christ, and all the blessings he delivers to your soul. If you're going to revel in and relish something, be like Paul:
May I never boast except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world. (Galatians 6:14)
Let the sorrows of your Savior on the cross move you. Imagine his cries and groans in your behalf, till your heart breaks. Daydream about how much love he showed you as he hung naked in your place. And see if the baits and lures of the flesh don't grow ugly and repulsive. Will you give your hours to fantasizing about and dwelling on and longing for the vile things that nailed the Lover of your soul to the cursed tree?
(Kris Lundgaard, The Enemy Within: Straight Talk about the Power and Defeat of Sin. P&R Publishing, 1998, pp. 96-97)
This little book has been a sharp and timely challenge to me. I recommend it to you!