Outside of scripture, I also like this Jewish quote:
"Evil concupiscence is at the beginning like the thread of a spider's web; afterwards it is like a cart rope." Sanhedrin, fol. 99.
Lust is at first a fine thread of temptation...like a spider's web...easily broken...not strong. But after the sin has fermented in the mind and lust has gained control it is like a cart rope forcibly pulling a man to sin. The secret is to never let evil thoughts control, but put them under subjection to Christ the instant they appear in the mind.
This also brings to mind another verse(s):
When tempted, no one should say, "God is tempting me." For God cannot be tempted by evil, nor does he tempt anyone; but each one is tempted when, by his own evil desire, he is dragged away and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death." (James 1:13-15)
"Any one who indulges in a sinful thought or corrupt desire, should reflect that it may end in death - death temporal and eternal. Its natural tendency will be to produce such a death. This reflection should induce us to check an evil thought or desire at the beginning. Not for one moment should we indulge in it, for soon it may secure the mastery and be beyond our control; and the end may be seen in the grave, and the awful world of woe."
Barnes' Notes on the Bible Commentary for James 1:15