R. Kruspedai said in the name of R. Johanan: Three books are opened [in heaven] on New Year,
one for the thoroughly wicked,24 one for the thoroughly righteous, and one for the intermediate. The
thoroughly righteous are forthwith inscribed definitively in the book of life; the thoroughly wicked
are forthwith inscribed definitively in the book of death;25 the doom of the intermediate is suspended
from New Year till the Day of Atonement; if they deserve well, they are inscribed in the book of life;
if they do not deserve well, they are inscribed in the book of death. - Talmud, Rosh Hashanah, 16B
The whole idea of a Book of Life used to scare me as a child. There was no way to know whether I've done enough Teshuvah or Mitzvot to deserve another year of life. There is no way to know G-d's Ultimate Plan.
The prayers of Rosh Hashanah does give us a formula to at least try to get us in the Book of Life: Teshuvah (Repentance), Tefillah (Prayer) and Tzedakah (charity). It is interesting that two of these requires us to connect with people in this world. We can't do full Teshuvah unless we apologize to the people we've wronged. By giving charity, we are obviously helping other people.
The root of the world Tefillah, is Pallel - which refers to a self judgement. While G-d is judging us, we are forced to take a real close look at ourselves.