The Mussar masters write that if a person were asked to sell a year of his life, or even one day or an hour of his life, he would disdain them—even if they would offer to pay with all the gold and silver in the world! But, if so, why does the person not wonder about his own behavior; how is it that he throws his life away on nothingness? Why does he let his days go up in smoke, or in laughter and levity, or in forbidden and sinful activities, such that his death would be much better than his life? Therefore, one who cherishes life will take utmost care that he not waste it, even more care that he has for great fortunes of gold and silver. He will constantly and consistently use every moment only for serving his Creator through either deed, speech or thought.
Serving Hashem in thought, which is to fear and love the great and awesome God, and to cling to Him and the like, is a constant type of service. One can do it at all times—while sitting and walking, and while lying down and when arising. When one is engaged in his business and preoccupied with his workers, or when involved in eating and drinking, a good and upright person never loses the opportunity to perform this service.
But a person who does not have the stamina to constantly focus his mind on only such pure thoughts, should review constantly those parts of Torah that he knows thoroughly and which he knows by heart, such as: the paragraph of Tamid-offering, the paragraph of Shema and the like; or he can review well-known Mishnayos such as “Yehudah ben Teima says…” (Avos 5:20), “Rabbi Chanania ben Akashya…” (Makos 23b) and the like, and he should review them even thousands of times. Our Sages long ago said (Tanna D’vei Eliyahu Zuta, 2) that even if a person repeats the verse Lotan’s sister was Timna (Bereishis 36:22) all day long, the Holy One gives him reward as though he had studied the hardest topics of the Torah. Each person is only responsible to do what he is capable of doing—that is his responsibility and there is nothing that exempts him from that. And although there is no man so wholly righteous on earth that he always does good and never sins (Koheles 7:20) in this matter, nevertheless, whatever is in the person’s power to do he should do, and “he who comes to purify himself is assisted [by Heaven].”