Read astaghfar all time time.
“Astaghfirullah al-lathi laa ilaha illa huwa, al-Hayyul Qayyum wa atubu ilayh” –
"Ask forgiveness of your Lord. Truly He is Oft-Forgiving. He will send rain to you in abundance; increase you in wealth and children; grant you gardens and bestow
on you rivers."'" [Nuh (71):10-12]
A man once came to al-Hasan al-Basri and complained to him: “The sky does not shower us with rain.” He replied: “Seek Allah’s forgiveness (i.e. say ﺃﺳﺘﻐﻔﺮ ﺃﻟﻠﻪ ).”
Also the below can be recited for Rain.
Allaahumma 'asqinaa ghaythan mugheethan maree'an maree'an, naafi'an ghayradhaarrin, 'aajilan ghayra 'aajilin.
O Allah, shower upon us abundant rain,
beneficial not harmful, swiftly and not delayed.
Abu Dawud 1/303. See also Al-Albani Sahih Abu Dawud
Source : www.Religioustutor.blogspot.in