One should renew repentance both when one remembers past sins and when when falls into them again:

(a) If one remembers a past sin it is recommended to renew one's remorse and seeking forgiveness, while being thankful to Allah for the success of leaving the sin; and

(b) If one falls into the same sin again it is obligatory to renew one's repentance, even if one keeps falling into the sin repeatedly.

The scholars caution, however, that one is not expected to try to bring to mind all one's past sins and repent from them. Rather, a general repentance from past sins is both valid and sufficient.

Some Key Issues Related to Repentance 

(1) One shouldn't leave repentance if one keeps falling into the sin. Rather, one should keep renewing one's repentance--while striving to fulfill its conditions of sincere remorse, leaving the sin, and resolving never to return. 

Often, sins are the manifestations of bad habits that have crept into our lives, and it takes repeated renewal of resolve to rid oneself of them. The Messenger of Allah (peace and blessings be upon him) reminded us in many hadiths that, "Allah continues accepting the repentance of a servant until they take their very last breath." [Tirmidhi (3460) and Ahmad (5885), from Ibn Umar (Allah be pleased with him)]

(2) The key to successful repentance is not merely resolving to leave the sin, but to figure out what is leading to the sin, and to take the positive means to stay away from the causes of the sin. 

So, for example, if one finds oneself watching impermissible things when using the internet alone at night in one's basement, then successful repentance would entail changing one's internet use such that the matters leading one to sin are avoided, while also fulfilling the conditions of sincere repentance (namely, remorse, leaving the sin, and resolving never to return).

The early Muslims used to say, "Whoever considers consequences is safe."

Sincere Repentence:

Allah Most High says, "Turn towards Allah, O believers, every one of you, so that you may be successful." (24:31)

And He says, "Ask your Lord for forgiveness and then turn in repentance to Him," (11:3) and He says, "O you who believe! Turn in sincere repentance to Allah." (66:8)

The Prophet (Allah bless him & give him peace) said, “The one who repents from sin is like the one who never sinned.” [Ibn Maja (3240)] 

He also said, “Remorse is repentance.” [Ibn Maja (4242) and Ahmad (3387)]

Abu Hurayra (Allah be pleased with him) reported that the Messenger of Allah (Allah bless him and give him peace) said, "Allah will turn towards anyone who turns in repentance before the sun rises from the place it set." [Muslim]

Imam Barkawi, the great Ottoman Hanafi faqih, grammarian, and sufi, defined repentance (tawba) in his Tariqa al-Muhammadiyya as, 
“Going back from desire to sin, with the determination of not returning to it, in exaltation of Allah and out of fear of His punishment.” [al-Bariqa fi Sharh al-Tariqa, 3.139]

The Conditions for Repentance

The conditions for repentance are well known:

1. Leaving the sin;
2. Remorse over having committed the sin;
3. Resolve never to return to the sin;
4. (If it relates to the rights of another person, then to) Return the rights or property one wrongly took.

[al-Bariqa fi Sharh al-Tariqa; Riyad al-Salihin]

If these conditions are truly met, then one can expect one's sins to be forgiven. However, one has to be very careful about how sincere one is in fulfilling one's conditions. It is recommended to seek forgiveness a lot, and to repent every time the sin comes to one's mind.

If one keeps repeating the sin, then one should find the root cause(s) of the sin and eliminate them. For example, if one falls into a certain sin because of the company one keeps, then it would be necessary to either stop keeping their company, or to change the nature of one's relationship with them.

So, what is true repentance?

Ibn Hajar mentioned that Qurtubi quoted 23 different definitions of true repentance (al-tawba al-nasuh) in his tafsir. Ibn Hajar mentioned the most important of these:

1. Umar’s words (Allah be pleased with him) that it is, “To sin and then never to return to it.” 

2. To hate the sin, and the seek forgiveness for it every time it occurs to one, as Hasan al-Basri (Allah have mercy on him) said. 

3. Qatada’s words (Allah have mercy on him), “To be genuine and truthful in one’s repentance,” which is what Imam Bukhari chose as the definition of true repentance in his chapter heading. 

4. To have sincerity in one’s repentance. 

5. To be concerned about one’s repentance not being accepted. 

6. To be such that it does not need another repentance after it. 

7. To be made out of fear and hope, and be accompanied by consistency in worship. 

8. Like the seventh, but with the added condition that one desert those who assisted one in sin. 

9. That one’s sin be between one’s eyes. [f: That is, one does not forget it.] 

[Ibn Hajar, Fath al-Bari]

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How is salaah al-tawbah (the prayer of repentance) offered? 

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By His Mercy to this ummah, He has prescribed for them an act of worship which is one of the best acts of worship, by means of which the sinner can draw close to his Lord with the hope of his repentance being accepted. This is salaat al-tawbah (the prayer of repentance) and there follow some issues that have to do with this prayer. 

1 – The prayer of repentance is prescribed in sharee’ah 

The scholars are unanimously agreed that the prayer of repentance is prescribed in sharee’ah. Abu Dawood (1521) narrated that Abu Bakr al-Siddeeq (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “There is no one who commits a sin then purifies himself well and stands and prays two rak’ahs, then asks Allaah for forgiveness, but Allaah will forgive him. Then he recited this verse: ‘And those who, when they have committed Faahishah (illegal sexual intercourse) or wronged themselves with evil, remember Allaah and ask forgiveness for their sins; — and none can forgive sins but Allaah — and do not persist in what (wrong) they have done, while they know’ [Aal ‘Imraan 3:135].” Classed as sahaah by al-Albaani in Saheeh Abi Dawood. 

Ahmad (26998) narrated that Abu’l-Darda’ (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “The one who does wudoo’ and does it well, then stands and prays two rak’ahs or four (one of the narrators was uncertain), in which he remembers Allaah and focuses well, then asks Allaah for forgiveness, He will forgive him.” The editor of al-Musnad said: its isnaad is hasan. It was also quoted by al-Albaani in Silsilat al-Ahaadeeth al-Saheehah (3398). 

2 – The reason for the prayer of repentance 

The reason for the prayer of repentance is if a Muslim falls into sin, whether it is a major or minor sin, he has to repent from it straight away and it is recommended for him to pray these two rak’ahs. And when he repents he should do a righteous deed which is one of the best of deeds, namely prayer. So he may draw close to Allaah by means of this in the hope that He may accept his repentance and forgive his sin. 

3 – The time for the prayer of repentance 

It is mustahabb to offer this prayer when the Muslim has decided to repent from the sin that he has committed, whether this repentance comes immediately after committing the sin or later on. The sinner must hasten to repent but if he delays it, it will still be accepted, because repentance is accepted so long as one of the following impediments does not occur: 

(i)                When the soul reaches the throat (at death). The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Allaah will accept the repentance of His slave so long as the death rattle has not yet begun.” Classed as hasan by al-Albaani in Saheeh al-Tirmidhi (3537).

(ii)              When the sun rises from the west. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever repents before the sun rises from the west, Allaah will accept his repentance.” Narrated by Muslim (2703). 

This prayer is prescribed at all times including times when prayer is disallowed (such as after ‘Asr prayer), because it is one of the prayers that are done for a reason, so it is prescribed when the reason for it is present. 

Shaykh al-Islam Ibn Taymiyah said: Prayers which are done for a reason, such as the prostration of recitation, greeting the mosque, the eclipse prayer, prayer immediately after purifying oneself, as mentioned in the hadeeth of Bilaal, and istikhaarah prayer should not be delayed if the reason comes up at a time when prayer is disallowed, because delaying them may cause one to miss them, such as if the one who is praying istikhaarah may miss the thing concerning which he is seeking guidance if he delays the prayer. The same applies to the prayer of repentance. If a person commits a sin, then it is obligatory to repent straight away, and it is recommended for him to pray two rak’ahs, then repent, as it says in the hadeeth of Abu Bakr al-Siddeeq. End quote from Majmoo’ al-Fataawa (23/215). 

4 –Description of the prayer of repentance 

The prayer of repentance is two rak’ahs, as it says in the hadeeth of Abu Bakr al-Siddeeq (may Allaah be pleased with him). 

It is prescribed for the one who is repenting to pray alone, because it is one of the naafil prayers that it is not prescribed to offer in congregation. After that it is prescribed to ask Allaah for forgiveness, because of the hadeeth of Abu Bakr (may Allaah be pleased with him). 

There is no report from the Prophet (peace and blessings of Allaah be upon him) to say that it is mustahabb to recite any particular soorahs in these two rak’ahs, so the worshipper may recite whatever he wants. 

It is mustahabb for the penitent to strive hard in doing righteous deeds as well as offering this prayer, because Allaah says (interpretation of the meaning): 

“And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them (till his death)”

[Ta-Ha 20:82] 

Among the best of righteous deeds that the penitent can do is giving charity, for charity is one of the greatest means of expiating sin. Allaah says (interpretation of the meaning): 

“If you disclose your Sadaqaat (almsgiving), it is well; but if you conceal them and give them to the poor, that is better for you. (Allaah) will expiate you some of your sins”

[al-Baqarah 2:271]  

It is proven that Ka’b ibn Maalik (may Allaah be pleased with him) said, when Allaah accepted his repentance: O Messenger of Allaah, as part of my repentance I shall give up my wealth as charity for Allaah and His Messenger. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Keep some of your wealth, for it is better for you.” He said: Then I shall keep my share in Khaybar. Agreed upon. 

To sum up: 

1-    This prayer is proven from the Prophet (peace and blessings of Allaah be upon him).

2-    It is prescribed when the Muslim repents from any sin, whether it is a major sin or a minor sin, and whether this repentance comes immediately after committing the sin or after some time has passed.

3-    This prayer may be offered at any time, including times when prayer is disallowed.

4-    It is mustahabb for the penitent to do some good deeds along with this prayer, such as charity etc. 

May Allaah send blessings and peace upon our Prophet Muhammad (SAWW) and upon all his family and companions.

Related:

Duas for forgiveness

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يا أيها الذين آمنوا
O you who believe, 

اتقوا الله 
be pious to Allah

وابتغوا إليه الوسيلة 
and seek the way of proximity to Him, 

وجاهدوا في سبيله
and strive hard in His way, 

لعلكم تفلحون
that possibly you would prosper. 

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[ سورة المائدة 5:35]