Accepting a job offer is a moment to celebrate, but sometimes factors come into play after you've accepted that may cause you to rescind your offer. Family problems, illness, better employment elsewhere or a last-minute relocation might all be reasons why you may need to cancel. Canceling can be tricky since you want to remain professional and not burn bridges with the employer in question but the most important thing to remember when revoking your acceptance is to be honest about your situation in an appropriate manner.
Choose a communication method that is professional and courteous. Once you formally accept a job, it is considered poor taste to cancel unless your excuse is extremely compelling, therefore it's important to pick the right kind of communication method to send the news. Canceling over email could be seen as too informal since you may have already interviewed in-person and met your employer face-to-face. A phone call would be the best way to cancel since it allows you to deliver sincerity in your voice and gives your employer a chance to ask questions of your predicament.
Candid answers are better than lies and will make you feel less guilty about canceling. For example, if a family member has suddenly fallen ill and you must take care of them full time, tell your employer exactly this. He will be more inclined to keep you in mind for a future job or recommend you to another employer once you're ready to work if he understands the situation at hand. If, on the other hand, you've been offered more money at different job, be candid about this as well. Not only will your employer probably understand since people typically interview at more than place, but he may also try to counteroffer you an even higher salary to get you to stay.
Follow up your reason for canceling the job offer with a sincere apology. Stress to your employer that it wasn't an easy decision to make and that you very much looked forward to working for him but unforeseen factors or offers suddenly cropped up. Your employer will be more willing to forgive your cancellation if he knows it was hard for you to turn down and may even keep you in mind for future positions.
Close your phone call on a high note. Do not end the phone call with another apology; instead speak about something you were looking forward to doing in your new position. Praise your employer, for example, on how revolutionary his company is in the field and express sincere regret for not being able to be a part of the team. Show him that you were genuinely excited about working in your new position and that you wish him well in all future endeavors.