How do I cut off bad thoughts before they take over?

In Orthodoxy we talk about constant prayer, specifically the Jesus Prayer, Lord Jesus Christ, have mercy on me, as a way of cutting off thoughts. Along with it comes an equally important practice: nepsis, watchfulness, watching our thoughts. Remember The Matrix, where the bullets come flying at Neo and he dodges around them? The saints tell us that when we begin to practice watchfulness, we first catch that the bullets are hitting us, and we know we've let the thought go too far.

But as we keep watching, the bullets get farther and farther away, and we catch them early, before they've gone into our soul and wreaked havoc and made a catastrophe out of a situation that is no catastrophe at all. You see this with lustful thoughts. Somebody used to entertaining them from their youth becomes a catechumen and says, I'm having so many lustful thoughts, what do I do?

The answer is always the same. Reject them with the Jesus Prayer, watch for them, and cut them off earlier and earlier, until the point where you see the thought coming in the distance and immediately say, Lord Jesus Christ, have mercy on me, brushing it away before it enters your soul.