Rise mightily against the first actings of thy distemper, its first conceptions; suffer it not to get the least ground. Do not say, “Thus far it shall go, and no farther.” If it have allowance for one step, it will take another. It is impossible to fix bounds to sin. It is like water in a channel,—if it once break out, it will have its course. Its not acting is easier to be compassed than its bounding. Therefore doth James give that gradation and process of lust, chap. 1:14, 15, that we may stop at the entrance. Dost thou find thy corruption to begin to entangle thy thoughts? rise up with all thy strength against it, with no less indignation than if it had fully accomplished what it aims at. Consider what an unclean thought would have; it would have thee roll thyself in folly and filth. Ask envy what it would have;—murder and destruction is at the end of it. Set thyself against it with no less vigour than if it had utterly debased thee to wickedness. Without this course thou wilt not prevail. As sin gets ground in the affections to delight in, it gets also upon the understanding to slight it.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, page 62