Fasten your seat belts, please.
Fasten your seat belt.
Buckle down your seat belt.
Fasten your seat belt, please.
Fasten your seat belt.
Fasten your seat belt.
Fasten your seat belt at all times.
Always fasten your seatbelt.
Fasten your seat belts while you are driving.
Fasten your seat belt while driving.
Please fasten your seat belts and prepare for departure.