Your unconscious mind sends a series of messages that you may not be aware of, which others can easily pick up.
"People read each other's intent as soon as they see each other," says Nick Morgan, speech coach and author of new book "Power Cues: The Subtle Science of Leading Groups, Persuading Others, and Maximizing Your Personal Impact." "We're hardwired to look for signals of friend or foe, to sense hostility, and to determine who's the leader."
What messages are you sending? Here are a few of the most common ways you can hurt your reputation at work without even realizing it:
Underpreparing for meetings
When you prepare for an important meeting, how much time do you spend thinking about what you're going to say versus how you plan to communicate that message with your body language? Most professionals spend 0% of their time on the latter, says Morgan, which can seriously undercut their authority.  
"Every conversation is two conversations — message and body language," he says. "When they're not aligned, body language always trumps content." If you're not aware of the second conversation, you may have done all your homework but come into a meeting slouched over, speaking in monotone, and conveying low energy levels. Your coworkers will likely focus more on your lack of enthusiasm than the ideas you present.
Using a "head posture"
You can win over or lose your colleagues in the first 30 seconds of meeting with them, says Morgan, just by the way you hold yourself. One of the worst body poses is something he calls the head posture, where your shoulders are rounded and your head is pushed forward. It's common in people who spend a lot of time in front of a computer, but it signals subservience to those around you.
"If your head is bowed over a smartphone, or you've got lots on your mind and your head is pitched forward, you look unhappy to be there," Morgan says.
Leaning back in conversations
Most functioning adults have learned how to control their faces and can easily portray a calm, interested expression, Morgan says. However, true feelings "leak out" in people's micro-expressions. "The body tends to carry out our unconscious desires," he says. And people are very good at spotting these signals in others.
One of the top ways you signal to your boss or coworkers that you're bored, impatient, or generally disinterested is by leaning back rather than into the conversation. Similarly, positioning your feet away from the person you're speaking with and towards the door indicates that you want to escape.
Not controlling your voice
"We completely underestimate the power of the voice," says Morgan. People with rich, resonate voices sound more authoritative and are more likely to become the leaders of a group. On the other hand, he says a thin, nasal voice is less appealing and often irritating.
Most professionals don't even think about their voices and haven't been taught how to control them. A stressful situation, like an important presentation or meeting, causes many to push their voices outside of their natural vocal range, which makes it sounds thin and weak. Women tend to go up too high, while men tend to go too low, he says. Sitting or standing upright, taking deep belly breaths, and letting your voice rise with passion and fall with authority help create what Morgan calls a "leadership voice."
Rambling
Another way that people squander their influence, says Morgan, is by not telling good stories. Research shows that good storytelling is powerful. It creates anticipation in the listener and actually synchronizes people's brains.
Unfortunately, most people are terrible at telling stories, he says. They ramble, provide way too much detail, and wind up for too long rather than choosing the right moment to start the story. "People vastly overestimate how interesting they are," he says. Great stories include conflict, grip the listener right from the start, and are stripped down to the most important details.