Answer:
The fact that you're getting interviews is a good sign. It means that your letters and resume are working and that you're making a good first impression. There are at least eight reasons you could be failing:

Your clothing is out of style. Make sure your clothing, including eyewear and briefcase, is current. Have shirts and blouses professionally cleaned and starched.

You come across as too much of a generalist, as someone who can "do it all"--jack of all trades and master of none. You haven't defined what you want or where you fit, and companies pick that up as lack of direction. Without sharp focus you appear scattered and may come across as a "loose cannon." To combat this, develop strong preferences and be clear about what you want and what you don't want.

In an attempt to "make the sale" you're pushing too hard, or coming across as desperate. You may appear too eager or overanxious. Create a high-impact, accomplishment-oriented resume and let it do most of the selling. In general, listen 75% and talk 25% of the time.

Who are you using to support your candidacy? Have you asked them what they'll say? Have you prepared guidelines for them? Do they have your resume? Do you brief them before they're called? Be sure to give employers references they can relate to. Engineers like to talk to other engineers, and attorneys prefer other attorneys. Last point: don't overuse your references.

Don't gauge your present worth on your last salary. The market may have changed; people with your skills could be in oversupply. Do a quick salary survey to determine realistically what you should be earning. Ask what the company plans to pay for the position. Then be flexible. You can lose out by seeming to care more about salary and benefits than about making a big contribution.

In multiple interviews companies have time to uncover weaknesses, character flaws, and problem behaviors, such as being arrogant or losing patience. You must appear co-operative, collaborative, and easy to work with.

Many companies hire by consensus. That means nearly everyone has to like you. Technical people often feel their track record "speaks for itself," but that's seldom true. In today's team-oriented environment, you need to make a strong effort to be liked by everyone you meet, from entry-level workers to the CEO.

Interviewing is a selling opportunity. It's a relatively short time frame and you're in the spotlight. Even in so-called casual interviews, you're watched and evaluated very closely. You're compared to others and graded. Everything you do, everything you wear, and everything you say is magnified, and either helps or hurts you.

Interviewers want to know at least three things: 1) Can you do the job? (Do you have the technical skills and experience?) 2) Will you do the job? (Are you motivated to perform?) and 3) How do you fit into the corporate culture? (Is the personal chemistry good?) To be successful, you need to win in all three areas. Let's discuss them separately.

Can you do the job? The company wants to know if you have the required technical skills and experience. They also want to know if you can take the ball and run with it. You want to show sureness (self confidence) rather than unsureness (lack of confidence). Don't lie, but don't be unnecessarily modest. You want to communicate "I can handle this with no sweat," not, "I could do it if you'd hold my hand every step of the way." Here are some good closing comments:

"The job fits me."
"This would be easy."
"I've done this before."
"I wouldn't have any trouble with that."
"We did a very similar project at AT&T."
"I could make a big contribution in a hurry."
"No problem. That's exactly what we did at Columbia/HCA."

Will you do the job? The company wants to know your level of motivation. Do you want the job? If so, how badly? (Remember that wanting it too badly can be interpreted as desperation.) Here are some closing comments:

"I could really see myself fitting in here."
"I think we'd work well together."
"I'd like the job."
"I'd like to take a shot at it."
"I'd love to take charge of this."
"I'd love to give it a try."
"I'd like to get started on it."
"It would be fun to get started."

How do you fit into the corporate culture? The company wants to know if you'll like others and if they'll like you. You want to use phrases that say, in effect, "I like it here." For example:

"I like you people."
"I like what you're doing."
"I like the direction you're taking."
"I like your management philosophy."
"I like what I've seen so far."
"From my perspective, it feels like a great fit."
"I like the way you manage people . . . and I'd like to work for you."

Footnote: A consultant came here to interview this week. Midway through the meeting I said I was going to get more coffee. She said, "Would you like me to get it?
I like to make myself useful." I had to laugh. She's a sales and business development person, and I knew what she was doing--she was selling me. I loved every minute of it, and told her so. We had a good laugh together. Her dropping that closing comment into the conversation really gave us something to talk about.

I'm convinced that if you drop these comments into your interviews, you'll make a wonderfully positive impression--good enough to get the job offer or consulting assignment you're after.