Trichotillomania and skin picking might not actually be related. Skin picking is often a sensory kind of thing, my ds would pick at the skin around his nails and hands until it was open and raw and oozing, because he was seeking sensory input, and because he doesn't really feel pain or discomfort. This is related to his Sensory Integration problems.
He actually had a severe anxiety problem when he was in school, too, but the skin picking didn't respond to the stuff we did for his anxiety, unfortunately. It can be hard to tease out what exactly is causing what symptom or reaction. Because it was his hands (and sometimes feet) that he picked at, and he seemed to get more satisfaction from picking at scabs and hardened skin, what I did was moisturize his hands and feet several times a day and cover them with cotton socks. This kept the hard skin from forming and also kept him from picking (at his worst I often had to tape the socks or gloves on his hands.)
I had a neighbor who was also hsing her son who had asperger's, and he had a serious skin and hair picking problem. Taking him out of school (since a lot of his anxiety revolved around school) helped to lessen the picking, but not entirely. For him it was totally anxiety based and dealing with the anxiety was the only way to stop the picking.
My ds was also a chewer, fiddler and general destructive tornado of a kid, who always needed some sort of sensory input, or he'd start going haywire. For him, constant redirection, and giving him things to fiddle with and bite (surgical tubing, koosh balls, sensory toys) helped a lot (he's also in the constant nose-bleed club
: btw Carrie, does your son have TS? My son does, and his sensory stuff and picking usually come at times when his ticks are starting a waxing phase.)
So, what I'm trying to say, in my long-winded kind of way, that how you approach this depends on where the behavior is coming from, if it's anxiety, or sensory-seeking.