What is the most important rule for descending a long, steep grade in a commercial vehicle?

On a long steep descent, choose a safe target speed and a low enough gear that the engine and any auxiliary brake (Jake brake, retarder) do most of the work holding speed. Service brakes alone overheat on long grades, leading to brake fade where the brakes become progressively less effective and can ultimately fail entirely. The rule of thumb is to select a gear lower than the gear you used to climb the hill, though modern engines often allow a slightly higher gear on the way down. Coasting in neutral on a downgrade is illegal in most states and extremely dangerous because you lose engine braking entirely and gain speed quickly. Using the parking brake to control speed will burn it up and leave you with no emergency brake. If service brakes overheat, use a runaway truck escape ramp without hesitation.