Be prepared to give the address and phone number of the location you are calling from. 

If calling from a cell phone, please do not assume that the 911 agency that answers your call will be the agency closet to you.  Most 911 cell calls are answered by a central communications center.  From there, the operator will ascertain the nature and location of the emergency and route the call to the appropriate agency.

Make every effort to stay calm and speak slowly and clearly so the operator can get the required information quickly without the need to have you repeat yourself.

Answer all questions the operator asks of you, even if you feel you have already given the information.  The operator may need to clarify information that could be confusing.  Asking clarifying questions does not hinder the response time.  While one operator is on the line with you, another is getting important information to the appropriate responding agency.

Do not hang up until the operator tells you do so, or if it becomes unsafe for you to stay on the line with the dispatcher.

If you call 911 in error it is important that you stay on the line to inform the dispatcher that you dialed in error.  If you hang up the phone without speaking to the dispatcher, the dispatcher is trained to send police to the computer given location that the call came from to confirm that no emergency exists.