There was once a Kollel fellow who learned with hasmada, and whose wife was moser nefesh for him to continue learning. One day, he felt that he could use a break, and mentioned that to his wife. His wife thought about it for a minute, and responded: "Your learning means everything to me. I take on countless responsibilities, so as not to waste a minute of time that you can devote to learning. My plan is to do my best to allow you to stay in learning for as long as possible. However, if you were to waste an entire day, I might reconsider whether my mesiras nefesh really makes sense. So, take your pick - whatever you do today is likely to be what you'll be doing for the long term."
The husband thought for a moment, and realized that it didn't make much sense to possibly lose the opportunity to learn full time for years, just to take one day off. Sheepishly, he picked up his Gemara and went straight to the bais midrash.
When the Yetzer Hara claims that he wants us to fall "just once", what he really wants is for us to get into a rut that we cannot get out of. Each time he presents us with temptation, we need to realize that what we do just this once might, c"v, be what we continue doing for the next 50 years, while living in utter misery. However, if we refrain just today, we can start "detoxing" and getting the garbage out of our system, which will give us a realistic potential to stay clean for the long term and live a meaningful life.