The Mishna in Pirkeh Abot (5:20) instructs us to be "Kal Ka'nesher" – "light as an eagle."
The Tur, at the very beginning of the work, comments that although this appears to refer to zeal and alacrity in the performance of Misvot, this cannot be the correct meaning, because the Mishna then instructs us to "run like a deer" ("Ratz Ka'svi"), which clearly speaks of swift, energetic performance of Misvot. Rather, the Tur writes, "Kal Ka'nesher" means that we must ensure to guard our eyes and avoid forbidden sights. An eagle, which soars to great heights and flies at great speeds, can see vast distances at every moment, and so this creature is associated with the faculty of vision. We must be exceptionally vigilant to turn our eyes away from forbidden sights, because sinful desires begin with the eyes – the eyes see something enticing, and then the heart wants it. The lure begins with the eyes, and so the Mishna urges us to swiftly turn our eyes away from things that we should not be seeing.
In fact, there is a tradition that if a person has the opportunity to look at an improper sight, and he turns his eyes away, then at that moment he can make any request from G-d, and the request is likely to be granted.
Of course, this is an especially difficult challenge, and, moreover, we often cannot control what we see, because things come into view without our having wanted them to. For this reason, King David prayed, "Ha'aever Enai Me're'ot Shav" (119:37) – that G-d should turn his eyes away from improper sights. We need to beseech G-d for assistance, and ask Him to shield our eyes from things that we should not see.
We read in the Book of Bereshit (17:5) that Hashem added the letter Heh to Abraham Abinu's name, which was thus changed from "Abram" to "Abraham." Hacham Baruch Ben-Haim explained that the name "Abraham" in Gematria equals 248, alluding to the 248 limbs in a person's body. Abraham was called "Abram" because he exerted control over his entire body except for five of his limbs – the two eyes, the two ears, and the nose. These are beyond a person's control, as he can encounter forbidden sights, forbidden sounds (such as Lashon Ha'ra) and forbidden smells (such as the scent of pagan sacrifices) without planning to. Once Abraham had fully mastered all that he was able to master, Hashem then added the letter Heh – stepping in to ensure that his eyes, ears and nose would likewise remain pure and be protected from anything improper.
If we exert the maximum effort that we can in this regard, and we beseech G-d for help, then He will give us the assistance we need to avoid seeing forbidden sights.