I
"You shall not stray (V'lo sasuru) after your heart and after your eyes" (Bamidbar 15:39). The heart and the eyes are spies for the body. The eye sees, the heart desires, and the body commits the sin (Rashi).
The Torah places the heart before the eyes, but Rashi reverses the order, stating that first the eye sees, and then the heart desires. Perhaps the heart does two things- one prior to seeing and a second, after. First, the heart strays[1]. As a result, the eyes stray, as natural curiosity takes over. This is then consistent with the order in the pasuk, as "The eyes follow the heart" (Medrash Tehillim 14:1).
Much of the expanded range of vision presented by natural curiosity is benign. Nonetheless, inevitably one's lust is aroused by what the eye sees, and at that point, the heart desires. Occasionally, a person cannot control these desires, and the body sins, as Rashi explains[2].
II
"'You shall guard yourself (vnishmarta) against any evil thought' (Devarim 23:10). A man may not gaze upon a beautiful woman even if she is unmarried" (Avoda Zara 20a).
Why isn't the violation of "v'lo sasuru - you shall not stray" mentioned here? The Smak (30) answers that "v'lo sasuru" applies only when one stares for the purpose of an immoral act. If one enjoys the beauty of a woman, but has no intention to commit an immoral act, he violates "vnishmarta". This distinction is reached independently by the Igros Moshe (Even Hoezer 1:69)[3]. However, the Mishna Berura (75:7) states that staring at a woman to enjoy her beauty is a violation of "v'lo sasuru". Perhaps this is the Rambam's position as well (see Sefer Hamitzvos, Lo Saaseh 47), that one who is pulled after physical lusts and a preoccupation with them violates v'lo sasuru, even if no sinful act is contemplated.
III
"If women are not properly dressed near a river, one who has an alternate route but chooses the river route is termed a rasha. If there is no alternative, he must force himself to avert his gaze"(Bava Basra 57a).
This passage has tremendous relevance nowadays, especially in the summer months. Men must avoid, if possible, walking in places where women are not dressed properly. When a man must walk in such a place to reach his destination, every effort must be made to avoid focusing on forbidden sights. Unfortunately, in most workplaces this vigilance must be maintained all day. (This vigilance includes avoiding prohibited internet sites.)
It is important to note that women are also included in the prohibition of v'lo sasuru if they gaze upon men with the intention to sin (Igros Moshe, ibid.). In addition, women may not wear clothing that reveals their upper arms or thighs (Mishna Berura 75:2), and certainly not any part of their torsos. The ubiquitousness of low-cut garments does not permit one to wear them. Tight-fitting clothing, which accentuates a woman's figure inappropriately, is strictly prohibited (see Az nidberu by Rav Binyomin Zilber). These laws reflect the requirement of tznius (modesty), as well as the interdiction of placing a michshol (stumbling block) in the path of men. In choosing their wardrobe, women must summon the strength to be discerning, and not to slavishly follow current fashion.
Other methods of arousing the male sexual desire are also forbidden, and can produce disastrous results. The Gemara (Yoma 9b) relates that women would entice young men by releasing perfume in their presence. This was a cause of the churban Beis Hamikdash. Even excess conversation, which can lead to levity, can be disastrous (see Avos 1:5).
In sum, both men and women must focus on resisting these behaviors, so typical of general society, and must govern their thoughts, sights, words/conversations, wardrobes, and deeds according to Torah law.
"If one sees a provocative sight (dvar erva) and does not allow his eyes to enjoy it, he merits to see the Shechina, as it is written, (Yeshaya 33:15 - 17) 'One who shuts his eyes from seeing evil shall dwell in heights and see the King in His splendor'" (Derech Eretz Rabba, 1). If we control ourselves, and do not stray after our hearts and eyes, we will be rewarded for keeping this difficult, yet critical, mitzva.
[1] One's heart should be totally dedicated to serving Hashem, as the Torah states, "You shall Love Hashem with all your heart" (Devarim 6:5). One who serves Hashem wholeheartedly is able to focus all of his senses in a single-minded effort to do good and avoid evil. If we allow our heart to stray, this ability is lost.
[2] The Gemara (Berachos 12b) states, "'After your heart' - this is heresy, akin to Avoda Zara. 'After your eyes' - this is sinful thoughts", in particular lust for women. At first glance, this statement is at odds with Rashi's interpretation which is found in Medrash Tanchuma. Rashi projects a progression: a straying heart leads to a straying eye. The Gemara refers to two totally separate sins - heresy and sinful thoughts. However, one may view a seemingly innocent straying heart, a lack of focus on loving Hashem, as a mild form of heresy, and even as containing traces of avoda zara. If one walks in the way of his heart and the vision of his eyes (Koheles 11:9), there is no law and no Judge (Medrash Raba). One who makes no effort to contain his thought inevitably sees prohibited sights. Carelessness in these areas reflects a lack of constant awareness of Hashem the Judge. This shortcoming smacks of heresy and avoda zara.
[3]This view is supported by the proof text of the aforementioned Gemara (Berachos 12b): Shimshon said to his father, "take her for me, for she is fitting in my eyes (Shoftim 14:3). Shimshon acted upon his lustful gaze and married a Plishti woman.