Although you may not be sure how effective will a thank-you note work, but it is definitely a very good tool for you to appreciate for the meeting as well as the extra information you want to deliver to the interviewer. Some employers may treat this as a show of interests from the applicants. A well-written thank-you note/letter may enhance your candidacy and express your gratitude for being granted an interview. Following is the guideline of the format:-

Make it short and clear!

Address the note to the person who will be most involved in the selection decision, or all if you are not 100% sure.

Ask for business cards during the interview and/or make notes about the spelling of this person’s name and correct title! You must spell the name of the organization correctly as well.

Use the note to confirm your serious interest in and/or qualifications for the position.

Detail relevant knowledge or experience to the position that was not covered or explained upon in your interview or on your resume / Mention people who were especially helpful during your interview / Reaffirm your excitement about this opportunity.

Maintain a professional tone

No matter how friendly a relationship you feel you have developed with the person to whom you are writing, the note will probably become part of your personnel file and be read by others.

Organize your note.

A suggested format:

• 1st paragraph:
  Express your appreciation. State the applied position and the date of the interview.
  
• 2nd paragraph:
  Personalize it! Mention something that enhanced your interest in the position or a skill/experience that you were not able to discuss during interview. Highlight some key reasons why you are qualified for the job and what the potential contributions you can make.
  
• 3rd paragraph:
  Make use of this space to clarify, if any concerns that the interviewer may have about you. Be sure to use some hard facts or past achievement to substantiate your points. You may not be able to deny a skill that is not good enough, but you certainly can show your attitude or commitment to improve it.
  
• 4th paragraph:
  Reaffirm you interest in the position. Tell the employer that you look forward to hear from them. Special thank you to the person who was helpful and support in this interview.

Send your thank-you note a day after the interview.
Do it as soon as possible. It will be too bad if your note arrives after the decision has been made.
If we do it right, a thank-you note is not only a courtesy, it will help set you apart from other candidates and enhance your chance to be selected for the organization.