Never, Ever Make This Mistake Negotiating an Offer There's one mistake you can make when negotiating a job offer that is much worse than any other -- and that's not knowing your worth.

Negotiating a job offer is challenging. There's pay, benefits and schedules to talk through -- and having so many details to discuss can leave room for a lot of errors. But there's one mistake you can make when negotiating a job offer that is much worse than any other -- and that's not knowing your worth -- and fighting for it, said David Wiacek, a certified professional career coach and resume writer.

"The single most common mistake I come across with my clientele -- mostly entry-level, and middle management, but, yes, even senior leadership -- is approaching it from a place of fear or desperation, which often results in folks under-selling and under-valuing themselves, and asking for less money than what they deserve," he said.

And of course, if you make the mistake of not knowing your worth, it "skews the entire process in the favor of the employer, and results in less earning potential," Wiacek explained.

So, why do so many people make this mistake? "The fear often arises from desperation -- as in, I hate my current job and need to leave, and therefore I should settle for whatever salary is presented without ruffling any feathers," Wiacek said. What's more, some candidates use their previous salary as the measuring stick for their new job offer -- but if your previous company underpaid its employees, that pay is a detrimental measuring tool, Wiacek says.

Related: How to Know if You're Underpaid

But don't worry: You can easily avoid making this mistake. Like anything else, negotiating a salary is a skill you can hone.

First, find out what you're worth. You can use Glassdoor's Know Your Worth tool to get an accurate estimate of what other professionals in your field and region are paid. Next, come up with a list of things you bring to the table that prove your worth beyond the market figures you uncover.

"Much of my client work is focused on getting clients to think objectively about the value they bring to the table," Wiacek said. "During an interview, and all the way through to the salary negotiation, your emphasis ought to be on what you can tangibly improve, mostly because you've improved things under similar challenges before."

If you can prove your worth throughout the process, negotiating your salary will be easier.

Then, practice negotiating! With practice, you'll develop confidence. And "showing that you are assertive and can stick to your guns, and you've done the industry market research, and you know your value, is compelling in its own right," Wiacek explained. "The assertiveness is a form of confidence, and employers want to hire confident, decisive employees. This skill will drive all sorts of success, whether internally focused [at the company] or externally."

Of course, if you've flubbed negotiating your salary and undersold yourself, all is not lost. Yes, it will be tough to backpedal and ask for more money, but "there is no rule book that says you can't do this," said Wiacek, who suggested approaching a re-negotiation by saying, "After careful consideration of the scope of this role, and what I've learned through the interview process, as well as additional research I've conducted into industry salaries, I'm confident that I can offer much more value to your company and that I'm worth $XYZ.'"

And, if you can't get a company to budge on salary, be sure to have a few other benefits in your back pocket that you can negotiate. "Maybe you'd value an extra week of vacation, or flex time, or working from home two days a week, or some other hard or soft benefit," Wiacek said. "Do the research, identify what's most important to you, and be willing to budge until you're not; in other words, have some hard limit after which you won't take the offer."