Quiet the Party Down
When the police knock on your door, you must quiet everyone down so that the police cannot determine how many people are there. If they feel it is a large gathering, and they suspect you of operating a tavern without a license, they may care more than if you have a few friends over.
Answer the Door Without Opening the Door
Most doors allow for conversation without being opened. Opening the door allows the police to see inside and make determinations that some other kind of criminal conduct is going on that allows them to enter without your consent. Also, do not let anyone leave. An ignorant party-goer could allow the police inside without your consent.
Do NOT Allow the Police Inside
If officers want to enter your house, tell them to get a warrant. They have no right to enter without your permission or a valid search warrant. Do not help them do their jobs.
Once the Police Have Left, End the Party
A repeat call will agitate the police and inspire them to use other kinds of tactics that you may not be equipped to deal with. Disregarding this step can lead to expensive tickets and nasty legal cases.