Lots of people use twigs. Sagebrush twigs in particular look pretty nice.
I've made a few hardwoods (in N) and I've never bothered to put anything on the sticks to keep them together. I guess you could soak them in diluted white glue or something, but unless you've got a bunch of HO monkeys swinging on them, they should be fine.
The best way I've found is to take the armature (sometimes a twig, sometimes twisted wire with a mixture of paint and dirt built up as bark), glue (rubber cement works) a couple chunks of cured lichen to the top, dip the top in diluted white glue, then lightly sprinkle ground foam onto the lichen. Darker stuff on the bottom, lighter stuff on top.
I've heard of using poly fill rather than lichen, but I've never gotten good results with it--probably because I tried to use the white stuff meant for stuffing quilts.
The biggest thing I've found is that each tree doesn't have to be amazing on its own. Plant a clump of them, put the nicer ones in front, and they'll look just fine. (I've made a lot of background trees!)
