How does one accumulate 100 blessings on Yom Kippur?
Every Jew is obligated to recite one hundred blessings each day. On Yom Kippur, due to the loss of several blessings from Shemoneh Esrei, and from lack of food and drink, one needs to accumulate these blessings from other sources. Aside for smelling spices, one is to concentrate on the blessings of the Aliyos to the Torah by both Shacharis and Mincha, and the Haftorah and answer Amen. [By doing so, one fulfills 28 blessings, and can easily reach a total of 100 blessings.] Nevertheless, one does not fulfill his obligation of Meiah Brachos with such a blessing unless he heard the blessing from the person saying it.
 Admur 46/1; M”A 46/8; Peri Chadash 46/3
 Dovid Hamelech instituted that one must say 100 blessings every day as a result of the sudden death of 100 Jews each day during his reign. At first Dovid did not know the reason for the sudden deaths until he saw with Ruach Hakodesh and instituted the one hundred blessings. This institution of reciting 100 blessings daily has Biblical support from the verse “What does Hashem ask of you but to fear Him“. The term “what” in Hebrew is “Mah” which can also be read as “Maeh” which means 100. Thus the verse is saying “100 [blessings] G-d is asking from you”. [ibid]
Kabalistic reason: The Ben Ish Chaiy [Balak] explains how the 100 blessings nullified the 400 evil Kelipos which caused the people to die. The Rokeiach explains that the 100 blessings recited nullify the 100 curses mentioned in Deuteronomy. [Kesher Gudal in name of Rokeaich brought in Ketzos Hashulchan 5 footnote 1]
Reason of Chassidus: The commands draw G-d’s radiance below in a concealed manner, in a way that it is not felt by the person. In order to internalize the radiance one is required to say a blessing prior to the command. It is for this reason that 100 blessings are required. As the number 100 in Kabala represents a level of G-dliness which is completely above the worlds, and it is this level which we want to reveal in our soul and thus unite with him. [Based on Sefer Hamamarim 5672 “Ayin Beis” volume 1 page 93]
Definition of day: The day in this regard starts from one night until the next night. [Admur ibid]