The generally accepted rule is at least 3 times, or seven, or ten. So make the appropriate duas at least three times after every fard prayers. When one has a pressing problem, one is thinking mostly of that problem. One can say the prayer 100 times, or even more. Or these can be made without counting. One can make dua all the time, except when in the bathroom.
1st Manner: Asking Allah alone
Allah (I) says:
“Do not make du’a to anyone alongside Allah.” (70:18)
Allah (I) says:
“Say (O’ Muhammad): I make du’a unto Allah only, and I ascribe unto Him no partner.” (70:20)
The Prophet (may Allah praise him) said:
“Du’aa<1> is worship”
Making du’a is an act of worship; directing, one’s worship to other than Allah alone is an act of major shirk.
2nd Manner: Praising Allah at the beginning of the du’a and asking Allah to praise His Prophet at the end of it.
Fudalah b. 'Ubaid reported that the Prophet (may Allah praise him) heard a man supplicating during prayer: the man prayed “O Allah, forgive me and have mercy on me.” So the Prophet (may Allah praise him) told him,
“You were hasty. When you pray, praise Allah, ask Allah to exalt my mention, then ask Allah.”
Another man praised Allah and asked Allah to exalt the mention of His prophet (may Allah praise him) so he (may Allah praise him) said:
“O supplicant, ask Allah and your prayer will be answered.”
Buraidah reported that the Prophet (may Allah praise him) heard a man saying, "O Allah, I ask You, I bear witness that there is no god but You, the One, the As-Samadh (the One upon Whom all creatures depend). He begets not, nor was He begotten; and there is none like unto Him." The Prophet (may Allah praise him) said,
"You have asked Allah by His Greatest Name. When one asks Him by this name, He gives, what one desires, and when supplicated by this name He answers." (Reported by Abu Dawud and Tirmidhi, who regards it a sound hadith)
Ibn Mas`ud (May Allah be pleased with him) said, "Allah does not accept du’a from the conceited, the one who flaunts and the one who is not serious, (He accepts only) from a person who praises and asks from his heart."
3rd Manner: Repenting to Allah
The person should repent for all of their sins and sincerely beg for Allah’s forgiveness.
4th Manner: Yaqeen (Certainty) and not being hasty
The person should be certain that their du’a will be answered. The Prophet (may Allah praise him) said:
“Ask Allah with certainty that He will answer your prayers, and know that Allah will not accept the supplication from an absent heart.” (at-Tirmidhi)
The Prophet (may Allah praise him) said:
“When you ask Allah, ask with confidence.”
Abu Hurairah also reported that the Prophet (may Allah praise him) said:
"None of you should say, 'O Allah, forgive me if You wish, (or) O Allah, have mercy on me if you wish.' Rather you should be firm in your request, for (Allah does whatever He wishes) and no one can force Him to do otherwise."
Some of the early Muslims used to say that the supplicant should be like a child when you ask Allah, crying until you are answered.
Abu Hurayrah reported that the Prophet (may Allah praise him) said:
“The person’s supplication will be answered unless he asks for sin or severing the ties of kinship, except if the person is hasty.”
Then the Prophet (may Allah praise him) was asked about hastiness. So the Prophet (may Allah praise him) described it as:
“The person says, ‘I asked, and I have not seen the answer.’ Then he leaves dua.”
`Umar (May Allah be please with him) said: "I do not carry the worry of acceptance but the worry of du’a. If I am inspired on how du’a is made, acceptance will accompany it."
`Aisha (May Allah be pleased with her) said: "No believer makes du’a and it is wasted. Either it is granted here in this world or deposited for him in the Hereafter as long as he does not get frustrated."
5th Manner: Asking Allah three times
Abdullah ibn Mas'ud reported that the Prophet (may Allah praise him) loved to repeat his supplication three times, and pray for forgiveness three times. (Abu Dawud)
The Prophet (may Allah praise him) said:
“If you ask Allah for the Jannah (The Heavenly Gardens) three times, the Jannah will say, ‘O Allah, make him enter Jannah.’ And if you ask Allah to protect you from hellfire three times, hellfire will say, ‘O Allah, protect him from the hellfire.’”
6th Manner: Asking in secret and supplicating in neither a loud nor low voice
Allah (I) says:
“A mention of the mercy of thy Lord unto His slave Zakariyyah when he cried to his Lord a cry in secret.” (19:2)
Allah (I) says:
“Neither say your prayer aloud, nor speak it in a low tone, but seek a middle course between.” (17:110)
Allah (I) says:
“Call on your Lord with humility and in private, for Allah does not love those who go beyond bounds.” (7:55)
Abu Musa Al-Ash'ari reported, "Once the people raised their voices in supplication. At this the Prophet (may Allah praise him) said:
'Be easy on yourselves. You are not calling upon someone deaf or absent. You are calling upon one who is All-seeing and All-hearing. He is nearer to you than the neck of your mount. O’ Abdullah b. Qais, shall I tell you a word that is one of the treasures of Paradise? It is: "There is no power, nor any might except with the permission of Allah'' (Muslim & Bukhari)
Abdullah ibn 'Umar reported that the Prophet (may Allah praise him) said:
"Hearts are like vessels, some more attentive and capacious than others. When you supplicate Allah you should be certain of being answered, and know that Allah does not answer a supplication that comes from a careless and inattentive heart." (Ahmad)
7th Manner: Tahaarah
It is preferred to have wudhu when making du’a.
8th Manner: Facing the Qiblah
It is more honorable to face the Qiblah when making du’a if possible. The Prophet (may Allah praise him) went out and prayed for rain and faced the Qiblah.
9th Manner: To ask Allah by His names and attributes
Allah (I) says:
“And to Allah belongs the beautiful names, so invoke Him by them.” (7:180)
10th Manner: Raising the hands
Salman narrated from the Prophet (may Allah praise him):
“Surely Allah is Hayyee, and He loves to conceal the shortcomings of His slaves. If His slave raises his hands, He does not let him go empty handed.”
Malik b. Yassar reported that the Prophet (may Allah praise him) said:
"Supplicate Allah while your palms are open and do not supplicate Him with their backs upwards."
Ibn 'Abbas is reported as saying, "When asking for something from Allah, you should raise your hands opposite to your shoulders; when asking for forgiveness you should point with one finger; and when making an earnest supplication you should spread out both your hands." (Abu Dawud)
11th Manner: Not consuming the Haraam (unlawful)
On the authority of Abu Hurairah, The Messenger of Allah (may Allah praise him) said:
“Allah the Almighty is good and accepts only that which is good. Allah has commanded the faithful to do that which he commanded the messengers and the Almighty has said: ‘O Messengers! Eat of the good things and do right.’ And Allah the Almighty has said, ‘O you who believe! Eat of the good things wherewith We have provided you.’”
Then he (may Allah praise him) mentioned
timbuktu says: However, these days we find oueselves overwhelmed with haram. At least have the intention to leave haram, and seek Allah's help in this too.
12th Manner: Doing good deeds and asking Allah by one’s good deeds
Qasim bin `Abd said: "I said to Anas bin Malik: 'O Abu Hamzah pray to Allah for us.' He said: 'Du’a is elevated by good deeds.'"
Bukhari and Muslim relate the hadîth of the Prophet (may Allah praise him) of the story of three men, who each supplicated to Allah by (mentioning) a good deed they had done earlier in their lives.
“Three persons of a people before you were on a journey when they were overtaken by a storm; therefore, they took shelter in a cave. A rock slipped down from the mountain and blocked the exit of the cave. One of them said, ‘The only way for deliverance is to beseech Allah and (to mention) some virtuous deed (he did purely for His sake).’”
One of the men mentioned a good deed which they had done for Allah, and supplicated, “O Lord, if I did this thing seeking only Thy pleasure, then do Thou relieve us of the distress wrought upon us by this rock.”
The rock moved, but not enough to free the men. So, the other two made similar supplications while mentioning their good deeds until the rock moved enough to free them.
13th Manner: Asking Allah in times of ease before hardship
The Prophet (may Allah praise him) said:
“Whoever is pleased that Allah answers his prayers during hardships and difficulties let him make much supplication during times of ease.” (at-Tirmidhi)
14th Manner: Knowing the best times for du’a
Some of the best times to make du’a are:
1) Middle of the night
The Messenger (may Allah praise him) said:
"The Gates of Heaven are open at midnight, and a caller calls: 'Is there a supplicant that his supplication may be granted? Is there a petitioner that his petition may be granted? Is there a distressed person so that his distress may be removed?' At that time, no Muslim makes a du'a but that it is answered, except for an adulteress who trades with her body, or a person who gathers his money unlawfully." [Sahih al-Jami' 2968]
2) At the time of athaan and between it and the iqamah
The Messenger (may Allah praise him) said:
"A du'a between athaan and the iqamah is not refused, so supplicate during that time."
The Messenger (may Allah praise him) said:
"Two kinds of du'a are not rejected - or rarely are they rejected - du'a at the time of the call for prayer, and du'a at the time of fighting when the warriors are on the battlefield."
3) On Friday after 'Asr
The Messenger (may Allah praise him) said:
"Friday has twelve periods. Among them is a period when no Muslim asks Allah for anything but He grants it to him. Search for that period late after 'Asr." [Sahih al-Jami' 8402]
4) When it is raining
The Messenger (may Allah praise him) said:
"Two kinds of du'a are not rejected: du'a at the time of the call for prayer, and du'a during rain."
5) i. A parents du'a for his child
ii. when one is fasting and
iii. while travelling.
The Messenger (may Allah praise him) said:
"Three types of du'a are undoubtedly granted: a parent's du'a for his children, du'a of a fasting person, and du'a of the traveler."
6) While Making Sujood
The Prophet (may Allah praise him) said:
“The slave is closest to his Lord while in sujood, so ask Allah in that time.”
7) Laylat’ul-Qadr
Aisha asked the Prophet (may Allah praise him) “What should I say on Laylat’il-Qadr?” And he replied,
“O Allah, indeed you are oft-forgiving, and love to forgive – so forgive me.”
15th Manner: Making du’a for others
The Prophet, (may Allah praise him) said:
“Whenever you make a supplication for another believer and he is not present, an angel will say ‘and same to you.’”
Abdallah ibn 'Amr ibn al-'As reported that the Prophet (may Allah praise him) said:
"The supplication that gets the quickest answer is the one made by one Muslim for another in his absence.'' (Abu Daw'ud and Tirmidhi)
16th Manner: Not making du’a against oneself or one’s children
The Prophet (may Allah praise him) said:
“Don’t make du’a against yourselves. Don’t make du’a against your children. Don’t make du’a against your maids. And don’t make du’a against your wealth. You might supplicate Allah during a time in which He answers the prayers.” (Abu Dawud)
17th Manner: Different types of people whose du’a is accepted
The Prophet (may Allah praise him) said:
"The supplications of three persons are accepted, and there is no doubt concerning their acceptance: the supplication of a father, of a traveler, and of one who is wronged.' (Ahmad, Abu Dawud, and Tirmidhi)
The Prophet (may Allah praise him) also said:
"The supplications of three persons are not rejected: the supplication of a fasting person at the time of breaking fast, of a just ruler, and of a person who is wronged. Allah causes their supplications to rise above the clouds, and gates of heaven are opened for them, and Allah says, 'By My Majesty, I will help you, even it be after a while'."
(Reported by Tirmidhi)
18th Manner: Memorizing the Authentic Supplications and du’as of the Prophet [Peace and Blessings of Allah be upon him]
We should all do our best to memorize the different du’as made by the Prophet (may Allah praise him) in the different situations throughout our day, to keep ours tongues wet with the remembrance of Allah, and by extension purify our hearts.