Candidates should formally accept or decline every job offer they receive. It is courteous to call the employer with your answer first to end the suspense, but then follow up in writing, with a formal letter accepting or declining the offer.
Your formal letter of acceptance should confirm all terms and details of the offer, including your start date, salary, and all other variables. If searches for other positions for which you have applied are still pending, it is customary to contact the hiring managers or search committees and withdraw from the running. Needless to say, if you have other pending job offers, you should waste no time in notifying those employers that you have chosen to decline their offers in favor of another.
Make an effort to stay in touch with the employer between the time you accept the job and your start date. You may wish to begin building relationships with your new colleagues by contacting them to introduce yourself. Perhaps you have a question they can help with. Familiarize yourself with your new employer as best you can before starting the job.
You may have to decline a job offer. Perhaps you receive a more attractive offer from another employer (or you anticipate that you will). Perhaps you decide that the position is not a good fit for you, or its terms are unacceptable, and you chose to take a different road.
Be very respectful, courteous, and polite when declining a job offer. In giving the employer your initial answer, and in your formal letter that will follow, be certain to make statements about whatever positive impressions the organization made on you. Let the employer know of your alternative plans, and keep in mind that professional circles are relatively small. You may run across these people again at some future point in your career. Leave a positive and professional impression on them.
Finding yourself in the fortunate situation of anticipating multiple offers presents its own problems and stressors. If you receive an offer from one employer but would prefer to hold out for other options, request more time from the employer making the initial offer (at the same time expressing how pleased you are by the news). Several days for decision-making is customary, but you may be able to negotiate for more time, depending on the pressures the employer is under. You may wish to inform your preferred employer(s) that you have received an offer from another organization (do not reveal the name). This information can sometimes expedite the process and result in an answer more quickly. It can also give you more bargaining power in negotiations. If, however, you cannot get results from your preferred employer within a reasonable timeframe, you may have no choice but to accept or decline the earlier offer without that information.