Note that these thank you letters should be written within 24 hours of the interview, and it is actually to your advantage to send it sooner rather than later. It could help sway the decision of who to hire in your direction, or at least to go to the next stage, which is probably a second interview.
The type of post interview and thank you letter depends on the company involved. If it a very formal company, then writing a formal thank you letter will probably be best. If the company is more informal, then perhaps a more personal hand written letter is better. Now not every one agrees on whether emailed thank you letters are acceptable or not. They do have the advantage of speed in getting there hopefully before a decision is made, and for companies that do a lot of business by email, this is probably fine. Even then, many advise sending a hard copy as well, just to be sure.
Now, although sending an interview and thank you letter is important, it is vital that you check and double check your grammar and spelling. It needs to be perfect! Get a friend to check it as well, just to be extra sure. You do not want to be remembered as the person who sent a thank you letter full of errors!
There are plenty of sample letters around, but do not copy them word for word. You need to put in your own content and customize these letters a little, and you should really send one letter to each person that interviewed you.
If you know that the hiring decision is to be made in the very near future, consider hand delivering your letter, if the company is close by. Otherwise you could send the letters by courier or express mail so that they get there before the decision is finalized.
Yes, it does take time to write interview letters and thank you letters, but it is worth the investment of time and the small expense, as it will get you remembered in the right way. Remember that in this thank you letter you can also reiterate your interest in this particular job with this particular company.