For flat trails, adjust your poles so your elbows are bent at a 90-degree angle.
Shorten your trekking poles to compensate for the incline.
Take pressure off your knees and prevent slippage by lengthening your poles.
At a slant:
For an extended traverse across a slope, lengthen the pole on the downhill side and shorten the other pole to give you more stability.
Taking a load off:
On short breaks, lengthen your poles, plant them about an arm's length in front of you, and bend at the waist so the upper portion of your pack rests on the tops of the pole grips.