seeking divine pardon is an antidote for the poisoning of human soul by sinning. every sin against allah, however heinous if may be, is forgiven if one seeks divine pardon for it. whenever a sin - great or small - is committed, it should be atoned for by seeking pardon, and even when no sin has been apparently committed, one should seek divine forgiveness. every muslim knows that the eternally blessed prophet muhammad was free of all sin, but even then he has said that "i beg for allah's forgiveness seventy times or more every day" (bukhari)

in one hadith the eternally blessed prophet is quoted as follows: "whosoever regularly seeks allah's pardon, allah makes for him a way out of all tight corners, removes all his worries and grants him sustenance from sources he had never imagined." (abu damd, kitab-al-satat, bab-al-istighfar).

hence one should cultivate the habit of seeking divine pardon at all times and at least once a day, one should tell a hundred beads of istighfar on the rosary.

سيد الاستغفار 

one can ask for divine forgiveness in any language. however, the arabic version is given below:

استغفر الله ربي من كل ذنب وأتوب إليه

one particular form of seeking divine pardon has been particularly lauded in hadith, and it has been designated as chief of all prayers for divine forgiveness:

translation: o allah! you are my sustainer, there is no god but you, i am your bondsman and am to the best of my ability firm on my oath to you. i seek refuge with you against whatever i have done.  whatever boons you have showered on me, i invoke them and turn to you and i also turn to you against my sins. hence forgive my sins because there is none save you who forgives sins.

hadith tells us that whosoever intones these words with full faith in the morning and dies before nightfall shall be counted among those of the paradise, and whosoever says them with full faith at night and dies before morning shall likewise go to paradise. (sahih bukhari, bab afdal-al-istighfar)

specially before going to sleep, one should briefly recall one's short comings and wrongdoings of the day and seek divine pardon for them and grace.