Register your demo with the U.S. Copyright office before approaching a record company. A Copyright protects your intellectual property and demonstrates to the label that you know what you are doing.
Research record labels to find a few companies that publish the kind of music that you play. If you're a rock band, don't waste your time submitting demos to a rap label. Target appropriate labels rather than mass mailing your demos.
Research those companies deeper to find the appropriate person -- a studio head or executive, for example - rather than sending a demo to the label's general address. This might mean the difference between your demo finding an ear and ending up in the receptionist's waste basket.
Track down and get to know a contact person so that your unsolicited demo might become solicited. You can also hire someone who can work with the record company. Making direct contact with someone at the record company requires good networking skills. If you hire an agent or an entertainment attorney, they may want to listen to your demo before they take you on as a client.