Q: How can one become more sincere?
A: You become more sincere by realizing (i.e. deepening and concretizing) your awareness of the One who is the only worthy target of your actions, Allah (God). This is done in many ways. Among them are the following:
* Reflecting on Creation (e.g. the sun, the moon, the stars, the sky, the water cycle, vegetation, animals, human beings themselves, your own body, etc) in order to comprehend what they indicate of Allah's Beauty and Majesty
* Reflecting on the afterlife (e.g. the Tribulations of the Grave, the Questioning by the Two Angels, the Resurrection, the Gathering, the Accounting, Crossing the Bridge)
* Performing your obligations
* Striving for mindfulness in your prayer
* Shunning bad deeds
* Mentioning Allah frequently
* Reciting and reflecting on the Qur'an
* Monitoring yourself scrupulously, both inwardly and outwardly
* Taking yourself to task
May Allah make us all servants of His who single Him out as the target of all our deeds and states.