It is much easier to keep the enemy at bay than to fight him once he has arrived.
Likewise it is easier to avoid seeing offensive sights in the first place than to repress the desire to stare at that which one has already seen, for the Gemara teaches: The yetzer hara controls only that which the eyes see.
The Midrash teaches that it was Chavah’s curiosity that led to her eating from the eitz hadaas. Once she was told that she should not eat from it, what was the point of looking at it? But she was curious, so she looked. And as the pasuk writes, She saw it was good ... and the rest, as they say, is history.
The problem is that people are by nature curious about their surroundings. So when we venture out of doors we tend to look all around, and much of what the street has to offer is not what we should be looking at.
What complicates this is that curiosity is certainly a good thing.
Our mind is trained to enjoy that which is new and fresh. Hashem made us that way so that we should explore and enjoy His Torah and Creation. As the Rambam writes, nothing engenders love and awe of Hashem more than the discovery of the wonders of
Torah and Creation. But like every other personality trait it has to be moderated.
An often overlooked harm that the Internet has wrought upon society - one that is completely unrelated to tumah - is that it has created “curiosity junkies.” The incredible access to information that the Internet offers allows one to get the “high” of discovery infinitely more often than was once possible. Just a click here and there - related sites are linked and they fight for your “eyeballs” - and one can discover things that he never knew before, and it can be quite intriguing.
It also serves as a dreadful distraction for whatever task one should be focusing on. Instead of working on something that may involve a fair amount of drudgery, as much of life demands, a person can instead receive a quick, novelty “hit” by clicking and distracting himself from his boring task at hand. This high, like all pleasurable experiences, can become addictive, and one can become a slave to his curiosity. How many people check their email while driving? The high of the “new” interferes with common sense.
Bear in mind the cautionary words of the Raavad, who writes:
Chief among all the safeguards is that one should guard his eyes from anything that is not his. If it’s not your business, why look?
The Raavad continues:
One should even guard his eyes from that which is his. He will then exemplify true modesty ... Should one guard his eyes, his heart will be guarded as well. And once his eyes and heart will be guarded, he will be entirely safe from sin, for one trans- gresses through three senses: sight, thought and touch ... (Baalei HaNefesh, Shaar HaKedushah).
The Chovos HaLevavos echoes this concept:
Work on shutting your eyes from looking at that which you don’t need, at that which will distract you from what is beneficial for you. Stay away, as much as possible, from too much looking, as Chazal tell us, “One’s heart and eyes are the two mediums for sin.
As a normal curious person, your “default” setting is to look around. Prepare yourself before you go out to the street by making a firm commitment not to look around.