Repentance could be defined as a feeling of being ashamed, sorry, and regretful for past sins. Some one who is indeed ashamed by heart for his past sins may truly be called as a repentant. The Holy Prophet (S.A.W.) said:
"Feeling ashamed and being sorry (for past deeds) is repentance.
- Haqayaq, p-286.
It is true that God-Almighty accepts repentance and forgives the past sins, but simply recital of the sentence: "I ask God to forgive (Astaghferullah), being ashamed and regretful or even, crying for past sins may not be considered as sufficient for a pure and sincere repentance, but with the existence of the following tree symptoms the repentance could be considered as true and realistic:
i. First: He must be disgusted by heart for his, past sins and his self
should have a feeling of shame, regret, and sorrow.
ii. Second: He must take a firm decision not to indulge into sinning in the future.
iii. Third: If, because of indulgence into a particular sin, he has done something, which could be compensated then he must take a firm decision for its compensation. For example: If he owes dues of people, has usurped property or stolen money, must decide to return it to its owner in the first available opportunity. In case he is not in a position to pay at present, he should try to get the owners consent or satisfaction through whatever means at his disposal.
If he has committed backbiting against some one, should seek his pardon, if he has oppressed some one, should try to redress the aggrieved. If religious dues have not been paid he must arrange for their payment, and if the daily prayers and the fasting have been missed, he must perform them as make-up (qaza) obligations. If some one has undertanen all the above steps, then he may truly be called as a sincere repentant, who is indeed ashamed for his past deeds and such atonement certainly receives God's acceptance.
But if some one recites the sentence: "I seek God's forgiveness upon his tongue, but by heart is not ashamed for his past sins, does not decide for avoidance of future sins and is not ready to compensate for those sins which could have been compensated - then such a person has not atoned and should not expect acceptance of his repentance, even though he might appear in a prayer assembly and, thus, being affected sentimentally might shed some tears or may cry loudly. A person recited the sentence: "I seek God's forgiveness in the presence of the Commander of the Faithful Imamn Ali (A.S.). The Imam said:
May your mother lament for you, do you know what is repentance? The repentance can be defined with the following six parameters;
1. Feeling ashamed and regretful for the past sins.
2. Taking firm decision for avoidance of sins forever.
3. Paying all the dues of the people so that when he meets God- Almighty on the Resurrection Day, he does not have any pending claim against him.
4. All the religious obligations (Wajibat) which have not been performed in the past should be discharged as makeup (Qaza) obligations.
5. Should feel so sad about his past sins that all the bodily flesh formed as a result of eating forbidden (Haram) should be melted in a manner that skin should touch the bare bones until the new flesh is reformed again.
6. The inconvenience and hardship of worshipping should b imposed upon the body as a compensation for the pleasures it faste because of past sins. Only after performing all the above, you may recit the sentence I seek forgiveness from God.
Wasail, vol. 11, p.361
Satan is so deceitful that sometimes he even deceives a persoi regarding repentance. It is possible that a sinner might attend a prayer gathering and after being effected sentimentally may shed some tears or may cry. Then Satan would say:
Great, wonderful! What a great thing have you done? You have already atoned and all your sins have been cleaned. While in reality, such person neither is ashamed from sinning by heart nor has be decided not to commit sins any more, and to remit dues of the creditors. Such act doe not constitute a real repentance and would not result one's attaining sell purification and eternal salvation. Such a person has not refrained from
sins and has not returned to God-Almighty.
Self Building; Ayatullah Ibrahim Amini
source : http://www.imamalinet.net