Tzitzis & the Heart
Torah taken to heart creates a spiritual light that shines within a person. Tzitzis has a similar effect, but on the body. Tzitzis creates a protective light around a person's body and even effects their thoughts. The Bracha on the Talis (the Arizal would make this Bracha on Tzitzis too) is "Lhisatef BaTzitzis", because the effect of Tzitzis is Misatef - it completely surrounds the body. This is why the Seforim say it's an Inyan to wear Tzitzis at night too - not to break this protection. The Mitzvah of Tzitzis corresponds to a fulfillment of all Mitzvos because its effect is all encompassing, on all parts of the body. The famous story of the man who was meticulous with Tzitzis who was saved from Aveirah in Menachos 44a supports this idea.
In these 2 ways we are protected from the thoughts of the heart, and avoiding this Lo Sa'seh is within our reach if we allow Hashem's Torah and the special Mitzvah of Tzitzis to enter our heart, our bloodstream, and completely envelop us. To the degree that we do this we will be protected - that is Hashem's promise.
The question I and many others have had, is how can I have impure thoughts if I learn Torah, if I know that Hashem is always watching? What happened to the special power of Torah to protect us from this? "Hamaor Shebah Machziro L'Mutav." Has all my Torah learning helped? Why am I still spinning with impure thoughts and actions?
The answer is that Torah does have a special power to keep us pure, but not if it only remains in the mental state, as thoughts in our mind. For Torah to protect us it must enter the heart, it must make an emotional connection. I think that's why Chazal compare Torah to a Tavlin - a medical remedy. Medicine has an effect when it enters our bloodstream - when the heart pumps it throughout the entire body. If it remains localized, it has little effect. If we connect with Torah emotionally, not just as a mental activity, then the medicinal powers of Torah will rid us of all impurities in thought, and consequently in action.