In today's world, an email thank you may be just as appropriate an a handwritten note.  Especially, considering the amount of time it takes. In our business, speed is a weapon. 

3 paragraphs. We suggest using the classic Opening, Body and Close…

1st. Thank the hiring manager about the time spent on the interview. If you can remember something unique about the hiring manager where you had something in common, or if you shared a story, a joke or a laugh, put it here.

2nd.  Share what you feel, based on the job description and what the company is looking for, your top 3 things that you can bring to the company. In other words, the top 3 reasons why you feel that you are the best candidate for the position, or why you think they should hire you.

3rd. Tell them your interest level and when you are available for the next interview, or if this is the final interview, that you want this job.
Remind them that you are looking forward to hearing from your recruiter on the next step.