Clean the fifth wheel, preferably using steam cleaning on the parts. This is particularly true of the top part.
Check to see that the fifth wheel mechanism is locking properly. Some operators prefer to just remove any old dried and caked grease from the mechanism with a putty knife before adding more.
Look over the fifth-wheel for any signs of grease going into places it does not belong.
Engage the locking mechanism with a test pin in order to see that it is still in good working condition. Brackets and pins should fit exactly with no signs of damage or excess wear.
Grease the locking mechanism with oil. Fill areas above the pins as needed.
Allow grease applied to the top plate to spread across the surface when the trailer is in operation.
