‘Tawbah’ refers to asking repentance from Allah for the sins that we commit. Making sincere repentance or Tawbah raises a Muslim’s stature with Allah.
One should renew repentance both when one remembers past sins and when one 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 Scholar, 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;
(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. [that is, one does not forget it.]
[Ibn Hajar, Fath al-Bari]
Source: Sunnipath.com
that following the revelation of this command of gratitude to Sayyidna Dawud (A.S), he submitted before Allah Ta'ala: 'O my Lord, how could I show my gratitude to You fully and sufficiently while my gratitude too, be it oral or practical, is nothing but a blessing bestowed by You? On this too, a separate gratitude becomes due.' Allah Ta'ala said, 
(Now, O Dawud, you did thank Me ). The reason was that he had realized his inability to thank Him as was His due, and had made a confession to that effect.
(Do good, O family of Dawud, in thankfulness) was revealed, the Holy Prophet (SAWS) came to the pulpit, recited this verse and then said, "There are three acts whoever accomplishes them would achieve the same excellence as was bestowed on the House of Dawud." The noble Sahabah asked: "Ya Rasulallah, what are those three acts?" He said, "Staying firm on justice in both the states of pleasure and displeasure; and taking the path of moderation in both the states of prosperity and adversity; and fearing Allah both in private and in public." (Tirmidhi, Qurtubi, Ahkam ul-Qur'an, al-Jassas)
, soon after having given the command for gratitude with special emphasis, the reality on ground was also pointed to by saying that 'And few from My slaves are thankful.' which is an admonition for a believer, and an encouragement to observe gratitude.