The Gemara in Meseches Sukkah (46a) states: "Come and see how Hashem's attributes differ from those of human beings. A vessel formed by a human being has the ability to store items only when it is empty. In contrast, Hashem's vessel has the ability to store only when it is full, as it is written, 'If you listened (to the Torah), you will listen,' but if you haven't listened [in the past], you will not listen [in the future].'" A person completely void of Torah is not a vessel that has the properties needed to retain Torah.
Rav Wolbe writes (Alei Shur vol. I pg. 201) that this idea is expressed by the Ramban in an insightful explanation of a pasuk in this week's parsha. The Torah tells us (Devarim 29, 17-18), "Perhaps there is among you a man or woman . . . whose heart turns away from Hashem . . . And when he hears the words of these curses he will bless himself in his heart saying, 'Peace will be with me though I walk as my heart sees fit', thereby adding the watered upon the thirsty."
The Ramban explains as follow: If one fulfills a desire to perform a transgression, even though his soul had previously been satiated ("watered") and did not desire sin; nevertheless, his soul will now "thirst" to transgress that sin once again. Moreover, his desire will grow to the point that he will begin thirsting to transgress aveiros that hitherto he had no desire for at all. As Chazal tell us (regarding immorality) the more one satisfies the desire, the hungrier it gets.
How does one assure that he doesn't fall into this terrible cycle described by the Ramban? This is accomplished through the performance of mitzvos. We were given 613 mitzvos and they dictate how we spend every hour of our lives. These mitzvos fill up our bodies and souls and prevent any unwanted desires from creeping into our minds.
There is no better way to prepare for the Yomim Nora'im than by making an extra effort to fill oneself up with extra Torah and mitzvos. An extra minute in the Beis Medrash or a short phone call to a lonely soul, does not garner merely the mitzvah at face value. It fills up our soul, thereby forming a vessel that can hold more Torah and mitzvos, and prevents unwanted thoughts from penetrating our hearts and minds.