When there is a need for water and there are no rains, then at such a time it is sunnah to make dua and ask Allah Ta'ala for rains. The mustahab method of making dua for rains is as follows:

All the Muslims should get together with their children, elderly people and animals, and walk towards the jungle in a very helpless and humble way, while at the same time wearing very simple clothing. They should renew their repentence, fulfil the rights of those upon whom certain rights are due, and should not bring along any kaafir friend of theirs. They should then offer two rakaats of salaah with jama'at without any adhaan or iqaamah. The imam should recite the salaah in a loud voice. He should then deliver two khutbahs just as they are delivered on Eid day. The imam should then stand up facing the qiblah and raise his hands and make dua to Allah Ta'ala for rains. All those present should also make dua. This should be done for three consecutive days and not more than that because this has not been established in the Shariah. If the rains begin to fall before they can go out to the jungle, or after offering this salaah for one day - even then the three days should be completed. It is mustahab to fast on these three days. It is also mustahab to give something in charity before going.