Candidates should formally accept or decline every job offer they receive. It is courteous to call the department 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, teaching load, and all other variables. If searches for other positions for which you have applied are still pending, it is customary to contact the 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 institutions that you have chosen to decline their offers in favor of another.
Stay in touch with the hiring department 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 department and institution 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 institution (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 department your initial answer, and in your formal letter that will follow, be certain to make statements about whatever positive impressions the institution made on you. Let the department know of your alternative plans, and keep in mind that academic circles are 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 institution but would prefer to hold out for other options, request more time from the institution making the initial offer (at the same time expressing how pleased you are by the news). The two-week period for decision-making is customary, but you may be able to negotiate for more time, depending somewhat on the time of year and the pressures the department is under. You may wish to inform your preferred institution(s) that you have received an offer from another employer (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 institution within a reasonable timeframe, you may have no choice but to accept or decline the earlier offer without that information.