There are, basically, three kinds of people: the unsuccessful, the temporarily successful, and those who become and remain successful. The difference is character.
The core of integrity is truthfulness. Integrity requires that you always tell the truth, to all people, in every situation. Truthfulness is the foundation quality of the trust that is necessary for the success of any business.
Steven Covey says that the key to earning the trust of others is to be “trustworthy.” Imagine that everything that you do or say is going to be publishes in the local newspaper.
A key part of trustworthiness is to always keep your promises. You should give promises carefully, even reluctantly, but once you have given a promise, you must always follow through on that promise.
From "How The Best Leaders" Lead by Brian Tracy