Premeditated: "Brace, Brace, stay down, stay low."
Unpremeditated: "Bend down, protect your head, stay down, stay low."
Try the unpremediated commands for yourself, say them to someone who doesn't know the correct brace position, sat in a seat. It's amazing how close they get to the correct brace position!
That was my idea!
Evac commands: "Unfasten seatbelts, come this way, NO handluggage."