You can find pack saddles rigged with both biothane and leather. Biothane is basically a rubber coated nylon webbing. It is cheaper, lighter, weather proof and rodents won't chew on it. I have a couple rigged in Biothane, but my Dad being on old school packer and saddle maker hates the looks of them. He calls them my Darth Vader saddles...

Leather obviously has the more traditional look, but requires oiling, gets stiff when cold and will hold sweat so rodents may chew them up. It is also heavier and more expensive. The obsolute best trees are the old OPR trees made by O.P. Robinette and are stamped OPR on the bars and were all made of Aspen or Cottonwood which is much less likely to crack than Pine or Fir. He made trees into the 1940's I believe and they are very hard to find anymore. He sold all his jigs to Ray Holes saddle shop in Grangeville, ID and they continue making some great trees.
To properly fit the bars to the animal, strip the saddle of all rigging, put some flour on the pack animals back and set it up there. You should have no high points. You should see a uniform layer of flour on the bars of the saddle. If not, bend dee's so that you get maximum surface area making contact with the pack animals back. You can also rasp any specific contact points of the bars down with a horse shoe rasp. Once the tree is fit, start with the the placement of the cinch. Bars of the tree should be up in the pocket behind the shoulder. A good starting point is to line up the front dee with the back of the front legs, and that should be pretty close. The back pad of the "spider" should be a hands width rearward from the top of the hip. Then move to the britchen, it should be two hands width below the point of the rump. It should also be angled slighlty upward in the front to "cup" the rumps when going down hills and to also match the slope of the rear leg. There should be a hands width of slack in both the britchen and the breast collar. A mule uses alot more of his arm pit than a horse does, so my general rule of thumb is 3/4 fire or less on a mule and 3/4 fire and above on a horse. I run my cinch at about 7/8 fire on a horse and 5/8 on a mule. There are a few books out there for packing. For deckers, Smoke Elser's book,
Packin in on Mules and Horses is a good one and for sawbucks, Joe Back's book
Horses, Hitches and Rocky Trails. Both will give you the basic hitches, but are a little outdated in my opinion.
Really watch for wear marks where your rigging lies on a mule or horse and adjust accordingly. There is no reason to be rubbing hide on these pack animals. Your general rule of thumb is to pack 1/5 of the animals body weight. That is the optimum load. The Army Corps of engineers determined this number early on and is good for 25 miles a day, 6 days a week. They can obvioulsy carry more, I have loaded 400 pounds on a mule before and Smoke packed a Grand piano into the heart of the Bob Marshall that weighed over 600 pounds but was only going about 6-7 miles per day.
It really all depends on how much packing you are going to do. If its just an occasional trip here and there, panyards are hard to beat whether on a decker or a sawbuck. I teach packing classes throughout the summer and you can find a lot of packing info on my site here
http://www.royaltine.com/forum/index.ph ... ,23.0.htmlGood Luck!