What is the correct practice for selecting a gear before descending a steep mountain grade in a loaded combination vehicle?

AAMVA CDL Manual (nhcdm.pdf) Section 2.16.2, p. 2-32; tested per 49 CFR 383.111(a)(19)

Brake fade from relying on service brakes alone is the leading cause of runaway truck crashes on mountain grades, which kill an estimated 50 to 100 people per year nationally. Selecting the right gear before the descent begins is the single most important decision a driver makes before heading downhill.