Rabbi Koretz explains that one should drink the rain water in the following manner: “Stand in the rain and reveal your head a little so that the rain falls directly on your head, also open your mouth so that the rain falls directly into your mouth from the right-hand side.
Additional sources of this spiritual remedy appear in:
1) In the Talmud in tractate Shabbat page 147b, it is written that “all potions taken between Pesach and Shavuot are beneficial”.
2) The Chatom Sofer explains this means during the month of Iyar as hinted to in the verse, “For I am Hashem who heals you”.
3) See the Holy Zohar on Parshat Metzora page 54a.
One does not need to recite a blessing on the rain drops that fall into ones mouth as one does not intend to quench one’s thirst.