While some people may disagree, I always prefer to send hand-written thank you notes as opposed to the more formal and dime-a-dozen typed thank you letters.  The reason for this is that I believe handwriting a card gives it a personal touch and shows that you put some thought into it.  With the amount of email and other electronic communication people see each day, receiving something handwritten stands out.  

When choosing what to write on, it's ideal to own a set of personalized stationary with your name on it, but not everyone can afford this or may need to send the note before there is time to order a set (Hint- Put it on your Christmas list).  A package of stationary or cards from a store such as Target or Hallmark is completely fine, just think classy and make sure you keep it professional by staying away from crazy colors and patterns.  When picking one out, choose a card that is blank on the inside so you have room to write everything you need to and don't have to work around the printed message inside.

How you address the recipient of the note is one of the first things they will see, so put some thought into it.  If they indicated that you should call them by their first name then address them by that, but if you're not sure go ahead and use Mr., Ms., or Mrs.  You will want to keep the note short and sweet, but at the same time fit in everything that you want to say.  Be sure to thank the person for their time right off the bat and reference the situation that you're writing the card for to refresh their memory.  To help figure out everything that you want to say, type your message on the computer first and once you've nailed it down transfer it to the actual note.  If you have horrible handwriting definitely go with sending a typed note; you want to make sure that everything is neat and legible.  Don't forget to sign the note, thank them one more time, and use a professional closing such as "Sincerely".  Also, add your contact information to the bottom in case they need to reach you.  After you're finished, read over the note to check for errors and then read it again or have someone else look at it just to double-check.

Writing a thank you card after an interview is a lot trickier than writing one for many of the other situations.  Since the interviewer will probably be receiving thank you cards from other applicants as well, you definitely want yours to stand out.  In this situation, it might be best to type up a more formal letter, just to ensure that you have room for everything you need to say and that it can be easily read.  

After the introduction where you thank them for their time, recap why you are the most qualified for the position and possibly give an example of how.  Don't leave any doubt in their mind that you have a strong interest in the company, especially after learning a bit more during the interview.  Touch on anything the interviewer brought up during the interview that you found especially interesting or anything that you connected over.  Also, use the opportunity to add in anything you might have forgotten to mention during the interview or feel you want to expand on, but don't explicitly state that you forgot.

Writing a thank you note should be done soon after whatever you're thanking the person for took place.  For example, if the note is for a job interview start working on it as soon as you get home and get it in the mail the same day.  You don't want to risk the person wondering why you haven't thanked them yet if they had some expectations about receiving a note.

It's generally not okay to send a thank you through email since this takes all of the personality and effort out of the process.  The only times I might send a thank you via email is if I already know the person well and sending a card might be a little over-board for whatever I'm thanking them for.  One exception is if you send an email and follow-up with a handwritten note.