The basic rule
Transmit your assigned call sign at least every ten minutes during a communication, and at the end of it. English is required for phone identification; CW and certain digital identification are also permitted.
You do not need to identify at the start of a transmission, though most operators do, and you never need to give the other station's call sign — a habit inherited from other services.
Self-assigned indicators
You may append an indicator such as /M (mobile), /P (portable) or /QRP, provided it is not confusable with a call sign prefix of another country.
Operating away from your licensed location
A US licence is valid anywhere in the US and its territories; you may simply sign your call sign, optionally with a location indicator.
Temporary privileges after an upgrade
Once you pass an upgrade you may operate with the new privileges immediately, signing your call sign followed by the appropriate indicator — /AG for General or /AE for Extra on phone — until the upgrade appears in the FCC database.
Club and special event stations
Club stations transmit the club call sign under a trustee. Special event call signs (1×1 format, e.g. W4N) are reserved through a coordinator for events, and the associated licence call sign must still be given at least once per hour.
Third-party and unattended stations
Repeaters, beacons and digital gateways identify automatically. If you are the control operator for an unattended station, its identification is your responsibility.