Interviews are crucial and unavoidable. If you ever work for someone else, assuming it's not a family member or close friend, you'll probably have to face one. But, since they're usually fairly infrequent occurrences, you won't likely ever get enough experience to be able to just walk in without preparing. Rather, you'd better do some serious homework before showing up. As much as anything else, your interviewer will be watching how gracefully you handle yourself under pressure. So be graceful.

While still at home
Research the company you're seeking to join. The more you know about what they do, their philosophy, history and competitors, the better you'll sound at the interview. For instance, if your research uncovered certain weaknesses in their business or market, you could tailor what you say about yourself to highlight your ability to help them overcome the problem. For instance, "in my last position, I was able to re-organize payroll systems and make them far more efficient."
Research (as much as possible) this particular job. How much might they be willing to pay? What exactly does the job require? Who was the unfortunate soul who had it last?
Research yourself. Take the work you've already done on your resume and prepare to expand on it. How will your employment goals and general lifestyle make for a good fit with this company? How does this particular job promise you personal fulfillment and pleasure (an interviewer wants to hear that you'll be happy: no one wants to hire someone who looks like he'll hate his job)?
Be prepared to use the fruits of all your research during the course of the interview.
You should also identify the person with whom you really should be speaking. Many companies (especially larger ones) employ people to save time and money by screening applicants - often making it impossible for you to actually speak to the one who really makes decisions. It's in your interest to slip past all those Human Resources types and get right to the Big Guy. "What Color is your Parachute" (Richard Bolles) includes some important advice on how to do that.
Marjorie Brody advises: "Think about your appearance. Select clothing that reflects your professionalism. Pay close attention to your grooming." Be very clear about your position on cross-gender handshaking. Consult your rav and, if necessary, work out a good excuse in advance. There is a near-universal expectation of a handshake at both the beginning and end of an interview. Make sure, whatever you do, that you don't fumble through it. Experts agree that it's crucial for you to try to anticipate all the questions that might come up during the interview. Whatever is going to interest the interviewer will probably fall into one of these categories: 1)Tell me about yourself.
Or, in other words, "can you help my company?" Be ready to fully address that question. Prepare honest answers designed to emphasize the kinds of qualities a business like this needs. Make it clear that you aim to solve their problems, not create new ones.
2)Why do you want to work here?
"Why here and nowhere else? Do you understand who we are and do you have the qualities to succeed with us?" Here's where your research into the company will pay off.
3)Why are you unemployed? (and other equally intrusive questions)
Your interviewer wants to know if there's anything seriously wrong with your work habits or personality. Response: stress your keen desire to work hard and the fact that you get pleasure from a job well done. You might have made mistakes, but you converted them to valuable lessons.
4)How much do you want to earn? (Salary Negotiations)
He's really trying to find out if you are unrealistic or priced out of his range (or if he can take advantage of your naivety). The correct reaction is to avoid giving any response at all (at least during preliminary, pre-offer interviews). Say "if it's ok with you, I'd rather discuss that later. I'm sure you pay generous salaries so it's not really an issue for me right now."

Immediate preparations
Make sure you know exactly how to get to the building (and to the interview office once you're inside) in advance so you don't arrive late. Similarly, arrange ahead for reliable babysitting etc., to cover for you while you're out. Dress as you would for the job itself; neither too formal nor too casual. Make sure to bring along any personal documents (ID) you might need.
Take a good look around the office for "props" you can use to start a friendly conversation. If there's a book on the desk you've read or some award or picture on the wall, see if you can't use them to develop some personal connection with the interviewer.

While you're there
Brody: "Wait to be seated. If the interviewer doesn't offer you a seat, ask where you should sit. Display quiet confidence. Sit up straight and tall in your chair, and look the interviewer directly in the eye. Don't fidget or squirm. Don't chew gum or smoke. Speak clearly. Keep your hands out of your pockets and away from your mouth.
"Keep your cool. No matter what the interviewer asks you, don't allow yourself to become annoyed or antagonized. If a question seems so inappropriate that you truly don't wish to answer, you can ask the interviewer calmly and politely how the answer to that question pertains to the position. If the interviewer persists and you are still uncomfortable, you can hold your ground by stating that you would be happy to discuss any information that would truly help determine your qualifications and interest in the position."
Bolles advises that you don't spend too much time talking, nor too much time passively listening: a 50:50 ratio is about right (and you should never talk for more than two minutes at one time). He also observes that, since most employers find it nearly impossible to objectively assess you in the short time they're sitting with you, their decision will depend more on emotional "chemistry" than on analysis. Your goal, then, is to make a good general impression; to generate chemistry. So think of the interview as a kind of date (you're there to answer the question: "can I really live with this guy eight hours a day?"), rather than viewing it exclusively as a professional evaluation (in which people are looking for technical problems).
When the interview appears to be coming to a close, don't forget to actually ask for the job. Most people probably take that for granted, but, it seems, this simple step can really make a difference.
What if the interview ends with a handshake and no suggestions for the "next step"? Politely ask the interviewer when you can meet again (or at least when you can call to find out if a decision has been made). In any case, always, always, always send a sincere thank-you note within 24 hours of an interview. You could include your contact information and briefly summarize your qualities and the job you were looking for. At the very least, your good manners might register with the interviewer. Besides the kiddush HaShem, that memory could usefully resurface sometime later.
By the way, while you're in the office, it's a good idea to spend at least a few moments to have a good look around and imagine how you - especially thinking of yourself as a ben Torah - might fit into the environment. How do other employees dress and relate to each other? Does the office seem to have an active social life (how might that effect your time there)? Are there any private spaces (or even semi-private cubicles) for davening/learning?

Salary Negotiations
Notice the wording: "salary negotiations." You've got to negotiate. Imagine what your interviewer is thinking: "this fellow wants a job where he'll be responsible for spending hundreds of thousands of our company's funds...and he won't even stand up to negotiate for his own salary"? In many cases, the money is just sitting there waiting for you to ask: few employers (or car salesmen) offer their bottom-line figure right away. At the same time you don't want to appear greedy or unnecessarily
aggressive. You hope to be working with these people on the same team for years to come, so don't act like you're ready to burn down the whole place for a few bucks.
So how is it done right?
Start by researching fair market value for such a position ahead of time. You will want to know what's reasonable and how much more the employer might be able to offer. Have a clear and realistic idea of your minimum salary needs (and of your more optimistic dreams) before you begin negotiations. Once you're at the interview, do nearly anything to avoid talking about salary until the right time (which is ideally the point at which you're confident that they want you but before they're sure you want them).
When the time comes, do your very best to get them to make the first offer - and never back down from a bid that you have made unless they've given a counter offer (good negotiators love to maneuver people into bidding against themselves).
Never, ever accept any offer right away. Sit silently and thoughtfully for a while, as though you're trying to absorb something that doesn't quite make sense. Ask "are you sure there's nothing else you can add?" Feel free to request a day or so to think it over. Just like any other important decision you'll make in your life, you can only gain by giving yourself a long, quiet evening to think and talk with friends and family. This is a complex and highly consequential agreement: make sure you can really live with it.
If you see that they really can't go any higher, demonstrate flexibility and creative thinking and consider alternatives to money. Try, for instance, remixing the benefit bundle. Remember, some combination of the following benefits can be equivalent to thousands of dollars extra in salary:
insurance,
stock options,
performance bonuses
commissions
pension and profit sharing plans,
vacation days,
paid holiday,
a shorter work week
sick/personal days
...so look for some compromise solution through which the employer can make up for a low salary ceiling.
Consider, too, discussing a 60- 90- day performance review after which, if your work is of a high-enough quality, your salary could be increased. This lessens the company's risk in taking you, but allows you to prove yourself and build towards an acceptable income.
Be willing to politely walk away saying "this job is really enticing but the only thing holding me back is salary." One thing, though, if you do walk away (or if you say something unconditional like "I would never, ever work for that salary") you have to be willing to accept the real possibility that the employer will simply sit back and watch you go. That happens (in fact it's happened to friends of mine).

What About Shabbos and Yom Tov (and other potential halachic restrictions)? If the question doesn't come up on its own, you should take the initiative and tell your employer exactly what conditions might exist that could seriously restrict your ability to do the job. Besides Shabbos and Yom Tov observance and things like kosher food at conferences and meetings, there are many potential conflicts between halacha and the modern world. Do your best to try to anticipate major problems before they become crises.
However, you needn't (and shouldn't) mention all these problems right away before the company gets a chance to become attracted to your many fine qualities. But don't leave it until after the deal is already struck. Important: be aware of your civil rights (at least as they exist in the United States):
(Title VII of the Civil Rights Act of 1964)
"(j) The term ``religion'' includes all aspects of religious observance and practice, as well as belief, unless an employer demonstrates that he is unable to reasonably accommodate to an employee's or prospective employee's religious observance or practice without undue hardship on the conduct of the employer's business."
(h) Notwithstanding any other provision of this subchapter, it shall not be an unlawful employment practice for an employer to apply different standards of compensation, or different terms, conditions, or privileges of employment pursuant to a bona fide seniority or merit system, or a system which measures earnings by quantity or quality of production or to employees who work in different locations, provided that such differences are not the result of an intention to discriminate because of race, color, religion, sex, or national origin, nor shall it be an unlawful employment practice for an employer to give and to act upon the results of any professionally developed ability test provided that such test, its administration or action upon the results is not designed, intended or used to discriminate because of race, color, religion, sex or national origin. It shall not be an unlawful employment practice under this subchapter for any employer to differentiate upon the basis of sex in determining the amount of the wages or compensation paid or to be paid to employees of such employer if such differentiation is authorized by the provisions of section 206(d) of title 29 [section 6(d) of the Fair Labor Standards Act of 1938, as amended].
From U.S. Equal Employment Opportunity Commission.

Source Material:
Besides Richard Bolles' "What Color is your Parachute?", this chapter also drew on ideas from the following sources:
The Four-Step Salary Negotiation Method by Kate Wendleton,
this site,
resume maker ("25 tips for Negotiating Your Salary"),
The Job Interview By Marjorie Brody, MA, CSP, CMC
And the pamphlet "Tips for Finding the Right Job" (US Department of Labor Employment and Training Division - 1996).

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