Allah, the Wise, has said:
أُدْعُونِــي أَسْتَجِبْ لَكُمْ
"Call upon Me, I will answer you." 
Imam Ali (a.s) said:
إِِدْفَعُوا أََمْوَاجَ الْبَلاَءِ عَنْکُمْ بِالدُّعاَءِ قَبْلَ وُرُودِ الْبَلاَءِ.
"Ward away, by means of supplication, the waves of misfortunes before the arrival of misfortunes (themselves)." 
The etiquette and conditions of supplication ought to be taken into regard and adhered to; the supplicant should carefully consider whom he is invoking and for whom or what purpose, he is supplicating.
The best of prayers is that man asks for obedience to Allah, drowning himself in His love and entrusting all his affairs to Him.
If Allah had not ordered us to supplicate and pray to him, but if we nevertheless had done so with sincerity, He, in His generosity, would have surely answered our prayers; accordingly, is it conceivable that the prayers of one, who adheres to the etiquette and conditions for supplication which Allah has prescribed, would not be answered?
When one invokes Allah, He either grants him his desire immediately, sets aside for him something better than what he had desired for, or wards away from him a great calamity.
1 – The Supplication of Mashlul
Imam Husain (a.s) narrates: "One dark night, my father and I were engaged in circumambulating the house of Allah when our attention was drawn towards a sound of weeping and lamentation. My father said to me: "O' Husain! Do you hear the wailings of a sinner, who has sought refuge in Allah? Locate him and bring him to me."
In the darkness of the night I went in search of him; finding him betwin the Rukn and the Maqam, I brought him before my father.
He was a young man possessing an elegant stature and dressed in expensive clothes. My father said to him: "Who are you?" He replied: "I am an Arab". My father inquired: "Why the wailing and lamentation?" He said: "Disobedience, sins and my father's curse have shattered my life and affected my health."
He (a.s) asked: "What is your story?" The youth said: "I had a father who had become old and had been very loving and considerate towards me; however I always used to indulge in inappropriate acts. Try as much as he would to guide me, I would not mend my ways and, at times, even troubled and inconvenienced him.
One day I intended to take his money, which he had kept in his trunk, but coming to know of my scheme, he tried to stop me. In the ensuing meli I flung him to the ground; he attempted to rise but failed. I took the money and set about my way, but on the way out I heard him say: "This year I shall go to the house of Allah and curse you."
For the next few days he engaged himself in prayers and fasts, and then started out for the holy Kaabah. I followed him and observed that taking hold of the Kaabah's curtain, he cursed me. He had hardly finished cursing me when I suddenly found that one side of my body had become numb and paralysed."
Having said this, the youth lifted his gown and pointed out his affliction.
He then continued: "I repented and sought his forgiveness. Thri years passed by and in the third year, during the season of Hajj, he agrid to pray for me. Together, we set off for Mecca and it was night when we reached the valley of Arak." In the darkness of the night a large bird suddenly took off in flight and startled my father's camel as a result of which my father was hurled to the ground and died. I buried him there but my affliction, due to my father's curse, still persists."
The Imam (a.s) said: "The supplication recommended by the Noble Prophet (s.a.w), shall come to your aid. It contains the Greatest Name (of Allah) and every miserable, sick or poor person who recites it shall have his desire fulfilled."
Giving him the supplication, he (a.s) said: "Recite this supplication on the eve of the 10th of Dhul Hijjah - the eve of Eidul Adha - and come to me the following morning."
As instructed, the youth approached the Imam (a.s) having been completely cured of his affliction. The Imam (a.s) asked: "How were you cured?" He said: "I recited the supplication on the eve of the tenth of Dhul Hijjah - all the while shedding tears of repentance. Having completed the recitation, I was about to recite it for the second time when I heard a voice say: "O' Youth! Enough! You have invoked Allah by means of the Greatest Name." I fell into a slumber and witnessed the Noble Prophet (s.a.w) in my dreams. He (s.a.w) placed his hand upon my body and said: "You have been cured." I woke up to find myself cured completely.'"
The supplication which Imam Ali (a.s) had taught the youth was the supplication of Mashlul, the initial sentences of which are as follows:
أَللٌّهُمَّ إِنِّــي أَسْئَلُكَ بِاسْمِكَ بِسْمِ اللٌّهِ الرَّحْمٌنِ الرَّحِيمِ. يَا ذَا الْجَلاَلِ وَ الإِكْرَامِ…
"O' Allah, surely I ask you with Your name, In the Name of Allah, the Most Gracious, 'O' The Lord of Majesty and Bounty".
2 – Congregational Supplication
Hafs Ibn Umar Bajalli relates: "Once, I complained to Imam Sadiq (a.s) about my abject financial condition and the dismal state of life whereupon he (a.s) advised: 'When you return to Kufah, take ten dirhams and prepare some food, even if it necessitates selling the pillow upon which you sleep, and invite some of your brethren-in-faith for a meal and request them to pray for you.'"
Hafs said: "I returned to Kufah and despite all my efforts I was unable to make arrangements for the money. So, in accordance with the instructions of the Imam (a.s), I sold my pillow, prepared some food, invited some Mu'minin for a meal and requested them to pray for the termination of my problems. After eating, they prayed for me. By God! Only a short period had passed since the incident when, one day, someone began to knock at my door. Opening the door I saw that the person knocking was one with whom I had previously engaged in trade and owed me some money. He had come looking for me and handed me a large amount - around ten thousand dirhams - thus, clearing off his debt to me. From that day onwards my work began to expand and my state began to improve till I was eventually drawn out of my poverty and relieved of my troubles."
3 – Repelling Calamities
The late Ayatullah Al-Hajj Sheikh Abdul Karim Haeri, the founder of the Hawzah Ilmiyyah of Qum recounts: "During the time when I was engaged in religious studies in the city of Samarra, an epidemic, in the form of plague, spread amongst the inhabitants of the city and everyday several people would die of the disease.
One day, some of the inhabitants of Samarra had gathered in the house of my teacher, the late Sayyid Muhammad Fisharki, when Ayatullah Mirza Muhammad Taqi Shirazi (d. 1338 ah), who, in terms of knowledge, was at par with the late Sayyid Fisharki, suddenly arrived. In the course of the conversation, the talk drifted towards the issue of the plague that had threatened the lives of all the people.
The late Mirza said: "If I were to issue a hukm (ruling), would it not be mandatory for it to be executed?" All those present said: "Yes." He said: "I rule that from today onwards, for a period of ten days, the entire Shiite community of Samarra should recite "Ziyarat-e-Ashura" and gift the rewards of this recitation to Hadhrat Narjis Khatun - the mother of Imam Mahdi (a.s) - in order that this calamity is repelled from them."
Those present in the gathering informed the other Shiites of this ruling and all of them engaged themselves in reciting the "Ziyarat-e-Ashura." The following days onward, it was observed that none of the Shiites would die due to the disease whereas the non-Shiites continued to suffer deaths - and this became plainly manifest for all the inhabitants of the city, such that some of the non-Shiites used to question their Shiite friends: "How is it that our people die due to the disease, whereas there are no deaths on your side?"
The Shiites would answer: "All of us recite "Ziyarat-e-ªshura" of Imam Husain (a.s) in order to remain protected from the epidemic and Allah wards away the calamity from us!"
4 – Supplicating for Rain
Once, during the time of Prophet Dawud (a.s), a famine struck. The people selected thri of their scholars, who procided outside the city to pray for rain.
The first of them pleaded: "O' Lord! You have ordered us to forgive one who has done injustice to us; we have done injustice to our own selves so forgive us."
The second one implored: "O' Lord! You have instructed us to set fri the slaves; we are your slaves so set us fri."
The third of them supplicated: "O' Lord! In your Torah you have commanded us not to drive away the poor and the unfortunate ones; we are the poor ones, who now stand at your door so do not kip us deprived (of your mercy)."
The supplications of these sincere and practicing scholars had hardly finished when Allah sent down rain over the people.
5 – Supplication for the Dead
When Bahiyah, a pious lady, was about to die, she raised her head towards the heaven and prayed: "O' The Lord, Who is my treasure! I place my trust in you; do not abandon me at the time of death and deliver me from the terror of the grave."
After she died her son used to visit her grave every Thursday night and Friday morning; he would recite Qur'an and supplications, and seek forgiveness for his mother and the other inmates of the cemetery.
One night, witnessing his mother in his dreams, the youth grited her and asked: "How are you?" She said: "O' My son! The intensity of death is indescribable but praise be to Allah, for I have been granted a beautiful place in the Barzakh."
The youth asked: "Mother, do you have any desire?" She said: "Yes, my son. Always continue to recite the Noble Qur'an, supplications and ziyarat for me. I am pleased when you visit me on Thursday nights and Friday mornings. When you come, the other deceased ones say to me, "Bahiyah, your son has come." This good news makes me and the other inmates around my grave happy."
The youth continued to recite the Noble Qur'an and supplications for his mother and the others when one night he witnessed in his dreams that a large group had approached him. He asked them: "Who are you?" They replied: "We are the inmates of the cemetery and have come to thank you for reciting the supplications and the Noble Qur'an for us. We request you not to abandon this practice of yours."
 Suratul Mu'min (40), Verse 60
 Bihar al-Anwar, vol. 10, pg. 99
 Tadhkeratul Haqa'iq, pg. 20
 Muhaj al-Da~wat, pg. 153; Dastanhai Az Zindagi-e-~Ali, pg. 191
 Baa Mardum In Guneh Barkhord Konim, pg. 107
 Dastanha-e-Shigeft, pg. 323
 Shanidaniha-e-Tarikh, pg. 22; Mahajjatul Baidha, vol. 2, pg. 299
 Muntakhab al-Tawarikh, pg. 849; Raudhatul Riyahin