I've long been a big fan of trekking poles. Besides helping to maintain balance and prevent slips and falls, I find they reduce fatigue. I think in rough terrain one uses a surprising amount of energy maintaining balance. When using a pole I find I can hike longer and further, and have more energy left at the end of the day.

I generally only use one pole. I think it is the old 80/20 rule. You get most of the benefit from the first pole. I generally carry two, but unless the terrain is really gnarly, one stays fastened to my pack.

My current favorite trekking poles are from Black Diamond and are made of carbon. They are extremely light. They also fold down to a very compact size, which makes them easy to pack in my luggage when I travel. The only minor disadvantages are that they are not adjustable for length, and the wrist strap could be better designed. And they are expensive, but I used one of the REI 20% coupons to get them down to a more reasonable price. Overall I really like them.