Take a hint from these hiring managers: First impressions count -- a lot.

Dave Ferber has had job candidates come in for an interview looking like they "just went through a wind tunnel," he says. Others have known close to nothing about the job they were applying for. But his biggest interview turn-off, he says, is still gum-chewing.

"All of a sudden you see them talking and you see a bubble and pop," says the director of human resources for the city of Ann Arbor, Mich. "One of the keys to an interview is making a [favorable] first impression, whether we like it or not. You don't get a second chance."

Mr. Ferber says he also had a candidate who asked midway through an interview: "Do you mind if I smoke?" He explained the local smoking laws to the candidate and continued the interview. "As an interviewer, it kind of catches you off-guard," says Mr. Ferber. He didn't hire the candidate.

One thing that bugs Dave Campeas is being put on hold during phone interviews while a candidate picks up another line. "You have to take a phone interview as seriously as a face-to-face interview," says the president of Princeton Search Group, a Princeton, N.J.-based network affiliate of search firm Management Recruiters International Inc., in Cleveland.

He also recalls a recent candidate for a sales position who asked an interviewer: "So, what do you do here?" The interview came to a swift end along with the candidate's chances. "You couldn't have said something more stupid," says Mr. Campeas. The problem with too many candidates, he says, is that they think, "I can just go in and wing it. I can sell myself."

Corporate staffing managers also suggest that job seekers be on their best behavior before they enter and after they leave the interview room.

Jeannie Mongiello, a vice president of talent acquisition for Prudential Financial Inc. in Newark, N.J., says one candidate for a senior-level legal position actually killed his chances between interviews. As her assistant escorted the candidate to a separate building, the candidate ignored the assistant and took out a cell phone and started making calls.

"That's a red flag," says Ms. Mongiello, who looks for candidates who demonstrate a talent for managing down as well as up in an organization. To gain a fuller picture of a candidate, she always asks how her assistant's interaction with a candidate went. In this case, she says, the business-group head who conducted the candidate's second interview "also asks that same question."