Reasons a babysitting job might be turned down include a prior obligation (birthday party, celebratory dinner, etc.) or a lack of available time due to schoolwork. It is also acceptable to simply say politely "I am sorry but I am not available to babysit on ..." and leave it at that (particularly if you hope the family will not ask again.)
Experts suggest it is best to avoid babysitting jobs in homes that are unsafe (ex. unsecured swimming pool), have unruly animals, or other hazards. Consider saying no to babysitting gigs where parents routinely come home later than expected and drink alcohol excessively.
Two examples of “babysitting dos and don’ts” from August 2012:
A 23-year-old man led Detroit law enforcement on a high speed chase which ended with a crash a foot pursuit. When the man fled the wrecked car, he took with him the 21-month-old he was babysitting.
A 15-year-old New Jersey babysitter was feeding a 3-year-old when the toddler began choking. The babysitter snapped into action, implementing the Heimlich Maneuver and saving the girl’s life. The life-saving measures she had learned 4 years prior came back to her in a flash. She encourages all babysitters to learn the Heimlich Maneuver.