“I Will Sin, and I Will Repent Later...”

Anyone who says that he will sin now and take advantage of his ability to repent later is prevented from doing teshuvah later. The Lev Eliyahu zt”l explains that this is not so much a segulah as it is a result of human nature. Since sin pollutes the soul and dulls its holy sensibility, the person who sins is not in the same state that he was before his sin. Beforehand, he had enough sensibility to know that what he was about to do demands repentance. Afterward, the sin affects him so that he no longer has the same resolve to repent. If before he indulged he failed to control himself, how can he possibly manage it now that his heart is blocked?
On the very last erev Yom HaKippurim of his life, a group of bochurim came to visit the Chazon Ish zt”l. One of the boys asked the gadol, “On Yom Kippur everyone does teshuvah, but didn’t we repent of the same flaws last year already? If we persisted in the sins that we repented of last year, and if it is likely that we’re going to fall into the same temptations again this year, doesn’t this coming teshuvah of Yom Kippur make us like someone who says, ‘I will sin and Yom Kippur will atone for me?’”
The Chazon Ish responded with a question of his own. “Why did Hashem create man with innate weaknesses? He did it for this very reason; that he will sin and repent, and sin again and repent again. This cannot be compared to a person who eases his guilty conscience by resolving to repent later on for a sin he is about to do. On Yom Kippur, we really do want to repent fully, and we sincerely take upon ourselves to change our ways. The one mentioned in the gemara who says that he will sin and will repent later does not really want to repent at all.”