Rav Wolbe writes (Alei Shur vol. I pg. 45) that had we not received any Mitzvos aside from Yom Kippur, from the Mitzvah of Yom Kippur alone we could deduce that the Torah is min hashamayim. Such an awesome day could not have been invented by man. The holiness of Yom Kippur is so great that it has the ability to obliterate the greatest sins and transgressions - if one desires wholeheartedly to rid himself of his wrongdoings. Even sins for which the desire for them is so great, that one can't even comprehend how he could not desire them, can be eradicated on Yom Kippur. One merely needs to sincerely yearn to be rid of them and believe that Hashem will assist him in his endeavors.
However, Chazal tell us (Yoma 85b) that Yom Kippur does not bring forgiveness for the sins that are between man and his fellow man, until he appeases his friend. If so, our avodah on Erev Yom Kippur must include appeasing those whom one has offended and asking for forgiveness. The Chofetz Chaim once spoke before Neilah and said that we can see the severity attributed to the mitzvos between man and his fellow man from the tefillah recited during Neilah, in which we beseech Hashem, "l'man nechdal mei'oshek yadeinu"; that our hands may refrain from stealing.
Most of us have someone or another that we have been meaning to appease but simply haven't gotten around to it. Now is the time. Make a call or send an email. It will be greatly appreciated, and your Yom Kippur will be more effective!
Gmar Chasima Tova!