When dialing 911, PLEASE try your best to do the following:
Speak as slowly as you can. You think you're being clear, and sometimes you're not. If asked to repeat yourself, it is for your safety-don't shout, just repeat as clearly as possible.
The operator understands that you are under extraordinary stress. In order to get help to you, she/he needs your exact location. If you don't know your exact location, give every minute detail that you can. Many tragedies have been prevented because the operator happened to know the "gas station where the garage is next door, and there's a bunch of tires stacked up...." Give what info you can, but be as exact as possible.
DO NOT feel the need to exaggerate the situation to get help there faster. Trust that the operator will dispatch the correct help as fast as possible. Tell the truth to the best of your ability. It may help your case if it ever goes to court.
Stay on the phone if you are safe to do so, until help arrives.
911 is for life/death emergencies only. You'd be surprised how many people think 911 is for noisy neighbors, barking dogs, and to find out the number for the pizza place (that's 411, by the way). Learn your police department's non-emergency line, and use it.
If you are calling from inside a building, let the operator know exactly where you are in the building. ("I'm in the second floor bathroom.")
Let the operator know if you have any pets with you. You don't want the police officers startled by your sweet corgi or pomeranian or tabby or boa constrictor, when they're coming into your house with guns in hands and their German shepherds at their feet to save you from a burglar. Trust me. That never ends well.
If you are disabled in any way, they need to know that too, for obvious reasons. If someone else in the house/building is disabled, they need to know as much as you know.
If children are with you, explain the situation as quietly, but truthfully as possible. ("There's a bad man in the house, and we need to be very quiet so he doesn't hear us. If he hears us, he might find us and hurt us. Do you understand?")
Most importantly, after giving your location, LISTEN to what the operator is saying. She/he is your lifeline, and will direct you to safety. The operator will tell you when the police/meds arrive, what they are doing, and when it is safe to come out of hiding, etc.