Futile Fasting

It is not sackcloth or fasting that cause one's prayers to be answered; rather repentance and good deeds cause one prayer's to be answered. For so we find regarding the people of Nineveh who fasted and prayed to G-d for forgiveness and were answered, that it is not stated, "And G-d saw their sackcloth and their fast", but rather, "And G-d saw their deeds that they repented their evil ways." (Yonah 3:10)

(Talmud - Taanis 16a)