How can one achieve sincerity of intention and maintain that sincerity?

Sincerity (Ikhlas) is a person's intention through their actions to seek the countenance of Allah the Almighty. The way to achieve it is for a person to know that Allah the Almighty alone is worthy of worship, and that any deed intended for other than the countenance of Allah is void and rejected. As Allah the Almighty says: (Whoever desires the worldly life and its adornment - We will fully repay them for their deeds therein, and they will not be deprived of it. Those are the ones for whom there is nothing in the Hereafter except the Fire. And all that they did therein has come to nothing, and what they used to do is worthless.) And in the Hadith Qudsi: "I am the most self-sufficient of partners concerning shirk (associating others with Allah). Whoever performs a deed in which he associates another with Me, I will abandon him and his shirk." Sincerity is achieved by abandoning hidden shirk, which is when a man "beautifies his prayer striving because he sees men looking at him." A good deed is that which is most sincere and most correct. That which is sincere is what is for Allah, and that which is correct is what is in accordance with the Sunnah. The servant should continuously check their sincerity and hold themselves accountable, for sincerity is precious.