Guarding one's eyes, like all spiritual growth, becomes easier as we go along.
The obvious reason for this is the famous principle, ַexternal behavior motivates internal character; for example, if we want to become kindhearted, we should act kind to others. If we want to become humble, we should act with humility.
This formula makes spiritual growth increasingly easy. The more we act in a certain manner, the more we internalize that character trait, and so we more naturally act in the proper manner as well.
This applies also to shmiras einayim. The more we guard our eyes, the more kadosh we become, and the more natural it therefore becomes to avoid looking at improper sights.
But aside from this, there is a less obvious reason for why guarding our eyes becomes increasingly easier.
We say in the third chapter of Krias Shema: Do not stray after your mind and your eyes.
The obvious question is: Why this illogical order? Do we not have to see something before our heart can desire it? Rashi says that this, in fact, is what happens: the eye sees and the heart desires. Why then does the verse refer to the heart before the eyes?
Rav Mordechai Gifter, the late Rosh Yeshivah of Telshe, Cleveland, explains: What you see is very much dependent on the direction to which your heart is turned.
To see this principle at work, ask ten people who have just walked down the same city block, what they had noticed as they were walking.
Chances are that a real estate agent will have noticed a "For Rent" sign; the young mother may have noticed that the children's clothing store is having sale; and a 10-year-old will notice a child in her age group walking down the other side of the street.
What each person sees is the direct result of what each person's heart desires.
If we think about what we notice in various situations, we can gain some powerful insight into where our hearts lie. Where do your eyes focus? On objects that possess kedushah or on those that possess its polar opposite?
If your eyes are always going in one direction, you can be sure your heart has preceded them there. But we can change. The more we actively guard our eyes, the more we focus on that which is kadosh, the more kadosh we in fact become. And as our hearts become kadosh, our eyes will naturally be drawn only to things that we should be seeing.
For example, R' Moshe Feinstein rules stringently with regard to walking past sights that are improper where there exists a "darka acharina" - another available route.
Someone once respectfully asked him why he himself took a certain route to yeshivah although there were immodest pictures along the way. R'Moshe responded that he literally had no idea what the fellow was talking about. He had never seen them.
We do not necessarily see everything in front of us. Just as our mind blocks out "white noise," things that we are uninterested in, and we hear only that which we wish to, so, too, is it with sight. One notices only that which interests him, that to which his heart draws him.