What makes someone a good interviewer?

A good interviewer:
1) is prepared for the session; read through the profile, prepare the questions
2) understand the position and job on hand; what is expected on the job, required performance level, immediate challenges, prospect within the company, etc.
3) has the ability to put the candidate at ease; so that he can speak freely with minial stress
4) is able to pick up the body language and the little gestures of the candidate during the process
5) share the truth about the current company situation and the demand of the positions; highlight the need to deal with difficult bosses, peers or subordinates, etc.
6) of course, i agree with the rest is the ability to listen and keep the conversation flowing, probing and ask further questions to evaluate the candidate better
7) make notes or summary about the candidate immediately after the interviewer and call for a discussion with the hiring panel
8) follow up with the candidate on an outcome be it positive or negative

To be a good interviewer one needs to understand the company and the department they are recruiting for. They also need to understand the needs of the position and what the manager/leadership is looking for and carefully crafting questions beforehand to see if the candidate possesses the qualities needed - not only in skill but in culture as well. By consistently passing along great candidates to hiring managers they will also earn their respect and their opinion on a candidate will be highly valued.

Listening is key! A good listener who can follow up on answers with additional questions will learn the most about a candidate and how they will perform in a company.

Interviewing is only one part of the recruiting process, make sure they understand all that is involved and can follow/create a plan.

Good question! A good interviewer must have a good understanding of the job, the type of person who fits into your company culture and your leadership style, He or she must be able to remove personal bias from the interview process and give you enough information so you can make an educated decision about the hire. The interviewer should be able to give you 3 outstanding candidates you can interview for the final selection. The interviewer you hire is out to please you, so you set the standards and the limitations. If the interviewer is afraid to take risks that could incur your displeasure, you will get candidates who have done your exact job somewhere else. In this economy, there is a lot of talent that might not fit the previous job but could add real value to your business. If you are open to creative hiring, make sure the interviewer has the opportunity to make a case for at least 1 less traditional candidate.

Listening carefully.

No matter the underlying reason for the interview, listening is key. We have all seen interviewers on TV that cut off response or answers often very rudely. That is completely unfair to the interviewee if nothing else.

The interviewer should let the interviewee say what they want and utilize what they say to gain more in-depth information. You will learn far more by listening than by talking.

First of all a good interviewer should concentrate more on the fact that what a candidate could be rather than what he or she currently is. It is very important to look for hidden possibilities In a candidate. There are three things which is very important for a interviewer to know about the candidate. First on scale of ethics how strong a candidate is. One can hire a expert in the required field but if the person is unethical then in the long run he or she is going to harm the organization. Second a good interviewer always look for the compatibility factor of the candidate with the organization culture. It is very important for a candidate to have a minimum compatibility factor with the culture of the organization. If a organization believes in horizontal structure of the management then bringing a candidate who strongly believes about the vertical structure of the management would be a total waste of time in recruiting the same candidate. Third very important factor is to look for candidate's Hidden potential. What is the belief of the candidate about himself or herself.
Knowing about the job knowledge of candidate is not very tough. Any field expert can find out but knowing the thought process of the candidate is very important.

Being able to listen what the applicate is saying and how to relate the answers given and how closely they are to the position interviewing.

Have you considered using a company such as http://www.thetestfactory.com/ to create bespoke online tests as an initial stage in your interviews? This may enable you to stay in more control of the process by creating shortlists for your interviewer. It also enables objectivity at this stage and has a great proven track record for choosing the candiadates that are best for the job not the candidates who are best at interviews.

Beside many that is mentioned above and with witch I mostly agree, there is also something that makes a big difference.
By my opinion, interviewer needs to have "god-feeling", that special inner "something" that will help him/her decide if candidate will be hired beside giving the best potential answers.
Candidates are coming more and more prepared to interviews, they are likely to give wanted/potentially good answer instead of honest one so we need to know and feel how to make a difference among these answers.
That is why I think that everyone can learn to be an interviewer but what really make a difference among average and great one is that special god feeling.