All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
As for sincerity in repentance, it is achieved when a repentant quits the sin exclusively for the sake of Allah, The Almighty, aspiring for His forgiveness and seeking to earn His pleasure and escape His punishment, as opposed to quitting a sin for a worldly reason. Examples of quitting sins for worldly reasons are quitting smoking or drinking alcohol out of fear for one’s health or that a drug dealer quits this sin out of fear of imprisonment or for his reputation. A repentant who quits a sin exclusively for the sake of Allah, The Exalted, has repented to Him sincerely, and he who quits it for other than Allah, The Exalted, is not sincere in his repentance.
Among the most helpful means to achieve a sincere repentance are: keeping the company of righteous people and distancing oneself from bad companions. Staying away from places that remind a repentant of sins or where he would be vulnerable to sin is another helpful means. In the story of the man who killed ninety-nine people, he was commanded to leave his town and move to another one where righteous people lived. The Hadeeth was cited in Saheeh Muslim, and scholars deduced from it that it is recommended for a repentant to depart the evil environment in which he disobeyed Allah, The Exalted, and shun his bad companions.
Moreover, among the most effective and helpful means to achieve sincere repentance is fear of Allah, The Almighty, evoking His Greatness and Glory, and reminding oneself that obedience is one of the due rights of Allah, The Exalted, over His slaves and that if it were not for His Forbearance and Will, no sinner would have been able to disobey Him. There are many means to evoke fear of Allah, the most important of which are: supplication, reflecting on His Names and Attributes, and contemplating His severe punishment and the disgrace and torment He prepared for those who disobey His commands. We ask Allah, The Exalted, to bless us with fearing Him both in private and in public.
Allah Knows best.