Walking downhill may seem to be a breeze, but it actually puts more strain on your knees and ankles as you hit the ground harder. Research shows that downhill walking has surprising benefits in lowering blood sugar.
Relax and Flow: Your stride will naturally elongate going downhill. This overstriding is bad on the level, but going downhill it helps to brake you a bit while gravity keeps you moving faster than usual. If you find yourself going too fast, shorten and/or slow down your steps and keep it natural.
Don't Lean Back: Leaning back will put you off balance, instead stay upright over your hips and knees or lean very slightly forward.
Your Knees: On steeper slopes, keep your knees slightly bent at all times and your torso upright or leaning slightly forward for stability.
Switchbacking: On very steep slopes or those with a loose surface, I like to take a serpentine path and angle for a few steps left, then a few steps right. This switchbacking is a common trail design to reduce steepness either uphill or dowhill.