Here's how to build unshakable credibility:
Never, ever lie. Never. Not only that, but go one step further. ...
If you make a mistake that reflects poorly on you, tell your boss before she asks about it. Human instinct is often to hide or soften this kind of thing, but, the truth is, the more blunt you are, the better you will come across. Just say it: "I really screwed something up." "I was completely wrong about this." Explain what you did, why you were wrong, and what you propose doing about it now. This also works in retrospect: "Do you remember how last month I argued for moving forward with that project when Bob insisted it was a bad idea? I was wrong. Here's what I've come to realize since then." Not only is this incredibly refreshing, it's also powerful because it instills in your boss the confidence that you will give her bad news directly -- she doesn't need to worry that she'll only get negative information if she digs for it.
Don't guess when you're unsure. Guessing means that some of the time you're giving out wrong information. Become known as the person whose information is always correct, and who just says so when she's not sure about something.
Do what you say you're going to do, by when you say you're going to do it (or update people accordingly). This behavior is so uncommon that if you do it 100 percent consistently, you'll become known for being reliable and keeping promises.
Don't let your emotions color your judgment. Stay as calm, rational, and objective as you can, even when you're frustrated or angry. For instance, say you have a difficult, irritating coworker who's always making suggestions that leave you with more work. It's easy to dismiss the suggestions as being bad or useless ideas, because you're annoyed. But you'll have more credibility if you assess the ideas honestly, acknowledging if they're good, despite your aggravation. (And when you have credibility, the objections you register are more likely be accepted.)
These behaviors tell people that you have integrity and that your priority is to be honest and objective, not to protect yourself or try to make yourself look good. And as a result, you'll find that your opinion will be taken more seriously, you'll get the benefit of the doubt in he-said/she-said situations, and, often, potentially contentious situations will go more smoothly.
Alison Green is the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results. She is chief of staff for the Marijuana Policy Project, a nonprofit lobbying organization, where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.