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Avoid saying "fair" at all costs.
Women are particularly prone to make this language mistake, says Lee E. Miller, co-author of "A Woman's Guide to Successful Negotiating" with his daughter Jessica.
"Research shows that when women negotiate, they are 'seeking a fair outcome,'" Miller told Business Insider. "Men do not. Men approach the negotiation as more of a contest, and their goal is to get the best outcome they can get. The concept of fairness is not an effective way to approach negotiation of salary because it's going to put the employer on the defensive."
There are two things that get you more money, Miller explains: when you have another offer or when your employer is afraid you might seek another offer.
"When you say, 'My salary is not fair,' that engenders a negative response," he says. "If you say, 'I've been approached by so-and-so and they're talking about a job where I'm getting x, y, or z, no one views that as pushy. Their response is, 'I don't want to lose this person, so what can I do to fix this?'"
"Fair" isn't smart language to use when you're negotiating compensation at a new job, either. One phrase you can use - assuming it's true - is, "I think this is below the market, can you reconsider?"
"It's 'no,' but it's not definitive," Miller says. "It's, 'Can you do better?' You have to always understand the employer is trying to recruit this person. They want you to start the job happy."
He recommends using phrases like, "Can you help me? Can you help make this offer one that will allow me to happily accept?"
"The key to all this is to give them reasons to want to help you, so you take the offer and take it excitedly," Miller says. "That's what we want. We don't want you not to be happy about it, because the first time someone comes along with a better offer we know you're out of here. If you have reasons for asking for other things, we'll probably say yes."