Although there is a specific mitzvah to do teshuva on Yom Kippur, we can't leave the mitzvah of teshuva solely for Yom Kippur. Such a fundamental aspect of avodas Hashem as is teshuva, writes Rav Wolbe (Alei Shur vol. II pg 438), cannot be accomplished in one day. It needs time, forethought and preparation.
Additionally, there is an impediment to the performance of this crucial mitzvah. While mitzvos that involve actions were given set times for their fulfillment, most mitzvos that are dependant wholly on the feelings of our heart were not allocated specific time slots. Hence, people tend to feel that the fulfillment of these mitzvos sort of happens by itself.
In reality no level of spirituality can be attained, "by itself." It is for this reason, that Chazal instituted specific actions to accompany mitzvos of our heart, lest we rely solely on the heart to perform the mitzvah. For example, even though one fulfills the mitzvah of renouncing his ownership of chometz with a mere internal decision that all his chometz should be null and void, Chazal required a physical activity in addition. They decreed that one must actually search for the chometz and destroy it.
This was not the case with the mitzvah of teshuva. Chazal did not institute any physical actions, and therefore, teshuva remains a mitzvah entirely in the realm of the feelings of one's heart. It follows that we must prepare our hearts to ensure a proper fulfillment of this mitzvah.
Rav Yisroel Salanter provides us with an effective manner of preparation. He writes that there is no better investment than the study of mussar. This together with hisboninus on how to improve some aspect of one's avodas Hashem, should succeed in enabling a person to make a kabbalah on Yom Kippur that will in some way change his future behavior - the primary goal of teshuva.
Yet, we must bear in mind that the kabbalah must be something small. Rav Yisroel Salanter offers an eitzah to help us facilitate the process of teshuva. He points out that the part of an averiah that is easiest to control is the most egregious part of the aveirah. He demonstrates this from Chazal who tell us that punishment for one who does not wear the white strings of tzitzis is greater than the punishment of one who does not wear the techeiles strings. This is because there is a bigger yetzer hara not to buy techeiles because it is more expensive, while the white strings are inexpensive and therefore an easier aspect of the mitzvah to fulfill. Similarly, for some, the punishment for not studying Torah on Shabbos might be greater than for not studying during the week when one is busy with his livelihood.
Our avodah during Elul should be to work on finding the easiest aspect of our avodas Hashem to rectify. This will enable us to make a kabbalah on Yom Kippur that will be very feasible to maintain, at the same time saving us from more severe punishment, G-d forbid. In this way we can reap substantial benefits from Elul and Yom Kippur.