The Job offer and the job interview
The awaited moment - YOUR OFFER. Then the D-Day comes and you have a company call you with GOOD NEWS. This is just great if you have applied to only one company. But if you have appeared for multiple interviews, this could put in a quandary. However, do not let dampen your spirits. Immediately make a list of the companies you have appeared for interview and then prioritize them in order of preference.
Now, you inform them in that order about the offer you got. This should be done in person, and not over the phone. This might call for some negotiations on the remuneration so be prepared for it. Sometimes, despite a very good offer, you might prefer another company. In that case, now it is the best time to indicate your preference. The company in question will love your interest in them, and you will seem even more precious to them since you were selected by another company.
Inform each of the other companies on your list. In case your present offer, decides you against any of those companies, inform them politely that you will accept the present offer, and thank them for considering you.
This information could be done through a hand-delivered note, or an email (followed by a telephone call to ensure that the message was received) informing them about the present state of affairs.
Paradoxically, when other companies find out that you are no more available, you will become all of a sudden 'hot property' and normally you will get immediate counter offers. Use this leverage to negotiate the best offer possible.