On the first day of Pesach, we recite during Musaf the special "Tikun Ha'tal" prayer, which marks the transition from "Mashiv Ha'ru'ach U'morid Ha'geshem" ("He who makes wind blow and brings down rain"), which we recite in Shemona Esrei during the winter months, and "Morid Ha'tal" ("He who brings down dew"), which we recite in its place in the summer. The first time we recite "Morid Ha'tal" is the Shemona Esrei of the Musaf service on the first day of Pesach.
If somebody forgot to recite "Morid Ha'tal" and recited instead "Mashiv Ha'ru'ach," and he remembered only after he concluded that Beracha (meaning, after reciting "Mechayei Ha'meitim"), he must return to the beginning of the Shemona Esrei. One must therefore be very careful during the first several weeks after we begin to recite "Morid Ha'tal" and remember to make this change.
If one cannot remember whether or not he recited "Morid Ha'tal," then during the first 30 days after the first day of Pesach he must assume that he followed his previous habit, and mistakenly recited "Mashiv Ha'ru'ach"; he must therefore repeat Shemona Esrei.
Halacha provides a method of avoiding this problem, however, whereby one establishes a new habit to recite "Morid Ha'tal." One accomplishes this by reciting ninety times, "Ata Gibor Le'olam Ado-nai Mechayei Meitim Ata Rav Le'hoshi'a Morid Ha'tal." By reciting this ninety times successively, one trains his tongue to recite "Morid Ha'tal" habitually after the sentence of "Ata Gibor…" Therefore, if any time from that point on he finds himself in doubt as to whether or not he recited "Morid Ha'tal," he may rely on his new habit, and need not repeat Shemona Esrei.
Chacham Ovadia Yosef rules that a person who makes use of this technique should recite this passage one hundred and one times, rather than just ninety times. The Talmud says that reviewing something one hundred and one times is particularly effective in ensuring retention, and therefore it is preferable to recite "Ata Gibor…Morid Ha'tal" one hundred and one times to guarantee the establishment of a new habit. Additionally, the rationale given for the ninety-time repetition is that one establishes a new habit over the course of a one-month period, and "Ata Gibor…" is recited three times each day, for a total of ninety times during the month. But as Chacham Ovadia Yosef notes, we must take into account as well the Musaf prayers on Shabbat, Rosh Chodesh and festivals, and one should therefore repeat this passage one hundred and one times, rather than just ninety times.
From the Shulchan Aruch's presentation of this Halacha it appears that one must repeat the entire passage, starting from "Ata Gibor," and it does not suffice to repeat ninety (or one hundred and one) times the words "Rav Le'hoshi'a Morid Ha'tal." The question thus arises as to whether one says G-d's Name when repeating the phrase "Ata Gibor Le'olam Ado-nai," or if he should instead say "Hashem," since he does not recite this passage as part of an actual prayer. Chacham Bentzion Abba Shaul (Israel, 1924-1998) rules that one may repeat this passage ninety (or one hundred and one) times with G-d's Name. Since one repeats this passage for the purpose of avoiding situations of uncertainty, this constitutes a legitimate need that allows for the utterance of G-d's Name.
Summary: We begin reciting "Morid Ha'tal" in place of "Mashiv Ha'ru'ach" in the Musaf service on the first day of Pesach. If one forgot to recite "Morid Ha'tal" and remembered only after the Beracha of "Mechayei Ha'meitim," he must repeat Shemoneh Esrei. If one is unsure, within the first month he must assume he followed his old habit and recited "Mashiv Ha'ru'ach." One can avoid this situation by reciting one hundred and one times the entire passage, "Ata Gibor…Morid Ha'tal," thereby establishing a new habit to recite "Morid Ha'tal" in place of "Mashiv Ha'ru'ach."
