I have come to realize that this is not only the key to success in work, but also in life. Mean the things you say, follow through on the promises you keep and if you can help it: don't be a flake.
At work I have learned to underpromise and over-deliver. Every time.
I will call a client on Wednesday to tell them that I have made no progress on whatever they have asked me to do and that they will hear from me again on Friday regardless of if I have an answer for them at that time or not.
At work I follow through. And if I don't you can be sure that that same client will call me out on it- I totally expect that. There is a great line from Jerry Maguire where one of the over-the-top-go-getter sports agents in a heavy southern accent promises, "If I don't return yer call in 24 hours, well, you can rest assured I am dead"- as Danielle Laporte says, I want that guy on my team!
Granted at work there is a level of professionalism that we naturally maintain, and in some ways it may feel as though standing behind what you say just comes with the territory. That's what you're being paid to do. But more and more I am also realizing how important keeping your word is in all of my relationships- and how empty promises, no matter how benign they seem, will erode any relationship. Period.
Of course you cannot always help going back on your word. We double book, we lose track of time, we forget about doctors appointments and sometimes we have to cancel. That's life. But check your intention when it comes to the words you speak. If you don't mean it, don't say it.
Silence will do wonders for your integrity every time.