An aspirant-to-be-successful asks: What is the mantra of successful people? My sincere reply: Sincerity, to be sure. As British Baptist preacher Charles Spurgeon said, "Sincerity makes the very least person to be of more value than the most talented hypocrite."
There is no way but to be sincere to be loved, followed and respected and finally what we call successful. The greatest 'weapon' Gandhi had was his sincerity of purpose which worked wonderfully better than any other weapon against the mighty British Empire.
Sincerity is a virtue that encompasses in it several other virtues like generosity, earnestness, kindness, positivity, willingness to give up selfish interests etc. That is why sincerity, with all its virtues, works as the surest tool to keep you confident and successful all through life.
Sincerity forms the very base of what we call 'dharma', which, in a very simple way could be defined as your truthful duty in life. Your duty is defined in terms of constructive activity that may or may not benefit you but has to be in sync with the welfare of the rest of the world. But there is no sincere deed that does not benefit the doer as well.
Just as sincerity is the key to a successful and happy life, it is unshakable faith that one has in the power of 'dharma' that becomes life's driving force. In fact, for those who are 'dharmic' in nature, it is impossible not to take the righteous path; and they don't take it that they are doing a great job or doing it for others.
For, they are guided by the principle of the purpose and meaning of life to which they stick very 'selfishly' as a guiding force through their journey on this planet.
Whatever you call it, 'dharma' or duty, it is sincerity that helps one comply with one's responsibilities in life in a manner that one gets the satisfaction of having done some purposeful work. And the realisation that it is sincerity that keeps one off the path of destruction and a meaningless life is by itself a great achievement.