Do not admit anyone claiming to be a repairman unless you have arranged for the repair. Replace the lock when you move to a new apartment, and don't leave your key in the lock. It takes only a second for a stranger in the hallway to snatch your key. Lock your doors and windows. Don't let strangers in to make phone calls.
Elevators can lead to trouble.
It is wiser to wait for an empty car than to get in with a stranger. Trust your instincts. It may be rude to leave an elevator or a situation if someone seems creepy, but it may save you from assault. If people make repeated anonymous phone calls to you at certain hours, it may mean they're checking to see if you're home. If you are bothered by such calls, or by obscene phone calls, notify the police and the phone company.