What to do after interview

Relax. You are not controlling the situation anymore. Try to get an experience from the interview: what did you make right, what was wrong. You may send a "thanks" letter, but I do not believe it really helps to change the mind of the employer. If you are not contacted during next day it does not mean you've been refused. Usually, employer has a time slot to interview as many candidates as possible. Other reasons include:

Manager left to vacation (and forgot about you when he came back)
Manager decided that they don't actually need anybody for that position

If you did not receive the answer during a week, you may call the company and to ask directly what are the results of the interview. Do not call every day! Sometimes you have a questionable discussion during the interview. If you feel you were right, you may send an email or a letter to the interviewer with explanation why do you think you were right. Again, I do not think it will help much. Major impression is formed during the interview.

When the company (or your recruiter) called and told you: "Congratulations! You've got the offer!" express you happiness, but never accept the offer immediately. You always have time to think. Recruiter will press on you, but do not believe him/her. It is your right and not obligation to accept/to refuse the offer. Once you've received the offer it becomes the best weapon for other offers. Like any multiple offer you feel like at auction: employers fight with each other who will give the most. Of course, if you cannot get another offer, you have no choice, but accept the first one. This is not fatal, but may be annoying if you don't really want that company or position.