Although it is perfectly acceptable to send the thank you letter in an email, sending a handwritten note is a great personal touch. If you were interviewed by more than one person, you should send separate thank you letters to each interviewer.
Be inclusive: Each person who interviewed you should receive his or her own thank you letter. You can never assume who is making the final decision, so including everyone will help you cover all bases. Also send a thank you letter to the recruiter who helped you, if you used one.
Be timely: Keep in mind that there are other candidates interviewing, and so staying in front of the recruiter or hiring manager is crucial. Sending your thank you letter in a timely fashion, within 24-48 hours is a good measure of time, and shows the employer your personality and desire to go above and beyond.
Look out for details: Choose your thank you card and stamp wisely. Something too fancy might give off the wrong impression. Likewise, using a Santa stamp in June will appear careless. The best type of card and stamp are plain, and do not express your personal or religious beliefs.
Stay professional: Remember, you aren’t thanking your friend. Maintain a professional tone throughout the letter. Avoid using slang overly familiar words.