How can a Muslim achieve piety in secret and avoid showing off, so that he may be sincere to his Lord and be mindful of Him in private, and thus not commit sins when he is away from people's eyes?

The path to achieving taqwa (piety/God-consciousness) is truthfulness, for in it is the sum of all goodness: truthfulness in intention and will (ikhlas - sincerity), truthfulness in resolve and fulfilling it, and truthfulness in actions by aligning the inner and outer self. Truthfulness also encompasses the stations of faith, such as fear, hope, contentment, and love, by the servant realizing their true meanings, not merely their outward manifestations. Truthfulness leads to Paradise; the Messenger of Allah, peace and blessings be upon him, said: "Hold fast to truthfulness, for truthfulness leads to righteousness, and righteousness leads to Paradise."