As has been said, the walking sticks or trekking poles help out a lot and let you get your upper body involved in the load bearing.
Every path to the ground is a path that you are distributing your weight to. Traditionally, this is done through the back, into the hips, through the legs, and into the feet.
Every single step, this adds up.
By using a walking stick or the trekking poles, you are creating another ground path (sorry, I'm an electrical engineer...) to carry the weight through. Your arms can "pull" the weight up a bit off your back, and sink some of that weight into the ground through the poles.
With one pole, it's every two steps, and with 2 poles, it's every step you see the benefit. It does add up at the end of the day.
Addressing Adam B.'s assessment, I went to REI and got a good pair for about $90. I am, however, not an idiot. I have a pair with a great handle, useable straps (this is KEY!), a camera thread bolt for use as a monopod, fully adjustable height (long uphill treks you can make them shorter and long downhill treks you can make them longer).
The hand strap is a critical aspect to the pole. You want the strap and you hand to do most of the gripping to keep the pole secure, not your actual hand and finger muscles. If you relied only on your grip, at the end of the day you'd have very sore hands, and at the end of your trip you'd have the best damn handshake of anyone. =)
The benefit of additional load paths to ground is lessened by the actual weight of the stick. So, for effectiveness's sake, the lighter you can make these, the better. This is where the extra $ for dedicated trekking poles comes in, as these tend to be far lighter than a wood walking staff or stick.
Finding cheap ski poles is a great way to save costs here, but you do give up some versatility. Make sure the poles are the right height. I've found that having them at pectoral height works for me so I can "pull up" on them a bit. Many people prefer having them set so their forearms are level with the ground.
There are concerns from enviro-weanies that the tips of the trekking poles digging into trails causes an increased erosion. If this is a concern you share, look at a rubber tipped type, or buy a few extra caps to keep on yours. I've even seen people mount tennis balls to their poles by drilling a hole in them and duct taping them on.