The first four Pesukim of this week's Sedra seem to contain a major redundancy. Moshe mentions the phrase "this day" three times, each of which could have been left out from his speech. Why does Moshe keep using this phrase?
In Moshe's final words to Israel he urges them to follow all the commandments of the Torah. They will be rewarded for doing so, but will be severely punished if they disobey the will of Hashem. However, there are moments when the temptation to go against the Torah are so strong that even the promise of reward and the threat of punishment may be insufficient to control one's behavior. Moshe teaches them that the key to success is "this day" (i.e. taking one day at a time).
There are things which are forbidden by the Torah that a person desires, to the point where he feels that as much as he'd like to, he cannot resist the temptation to sin. He reasons that since eventually he will give in to the urges to sin, he might as well not fight the urges at all, and sin willingly. Moshe is teaching us that the best way to deal with the Yetzer Harah is to deal with temptations as they come up, and not top worry about faltering later on. One must try to fight his temptations today! There is nothing to do about tomorrow's temptations which will surely arise. One should try to deal with the temptations as they come up. After all, if someone feels that he doesn't have the strength to deal with his temptations for the rest of his life, he can still fight them on a day to day (or hour to hour) basis, with a day representing a smaller, and therefore easier challenge to handle. The Yetzer Hara wants us to look at the big picture and feel depressed about lost opportunities and past sins. However, by realizing that we cannot change the past, we can avoid depression over it, and focus on the future, a time where we can still achieve greatness.
By concentrating on daily temptations as they arise and avoiding depression over past mistakes, one can follow the commandments of the Torah with more ease and happiness.