To prevent a trailer skid (and possible jackknife), what is the most important driving practice?

To PREVENT a trailer skid/jackknife, the most important practice is SMOOTH driving with GRADUAL braking — avoid over-braking and hard braking, which can LOCK the trailer wheels (locked wheels lose traction and the trailer slides out). CDL combination. ESPECIALLY on slippery roads and in curves. Brake BEFORE curves, not in them; slow down for conditions; maintain space to avoid sudden stops. EMPTY/lightly loaded trailers are MORE prone to wheel lockup (less weight on the wheels). If a trailer skid begins, RELEASE the brakes to let the trailer wheels regain traction and roll (which brings the trailer back in line). PREVENTION (smooth, gradual braking) is far better than recovery. Knowing that smooth/gradual braking prevents trailer skids is commonly tested.