Desirable and Accepted
Even though repentance and calling out to G-d are desirable at all times, during the ten days between Rosh Hashana and Yom Kippur, they are even more desirable and will be accepted immediately as Isaiah 55:6 states: "Seek G-d when He is to be found."
(Rambam)
The Urgent Matter
A merchant, one of the chassidim of the Sefat Emet, came to him during the days between Rosh Hashanah and Yom Kippur to seek advice about his business. As was customary, he submitted a note and added verbally that the matter was extremely urgent and that he needed an answer that very day. The Rebbe replied, "today, the urgent matter is to do teshuvah."
What About the Mitzvot?
One of the chassidim of the Maggid of Mezritch once met a fellow chassid and said, "my brother, what will become of us because of all our sins?" His friend re-plied, "I am not concerned about the sins, because there is teshuvah in this world. But what will become of our mitzvot? With what face can we stand before the King of Glory with mitzvot like ours?"
Don't Pat the Yetzer Hara
The chassid Rabbi Shmuel Betzalel would say: When we recite ashamnu, we should strike our chests not with our hand, but with a dry stick. When we strike with our hand, the yetzer hara thinks we are patting it affectionately.
Now We Begin
The Rebbe Rayatz related, "once, on the day after Yom Kippur, I entered to my father (the Rebbe Rashab) and asked him, 'what now?" "He replied, 'now we have to begin doing teshuvah."