c.sam, in his excellent book on model railroad scenery, Dave Frary devotes a section to gathering, cleaning, sorting, dying, etc. your own lichen, including the glycerin treatment.
I'm not much of a scenery expert, but I have used pre-processed lichen because I liked the natural branching, that the foam products can't match. That said, I have been gradually coming around to Frank's view, that lichen has pretty much been surpassed by modern materials.
The hand gathering and cleaning of lichen is extremely labor intensive, and wasteful: according to Frary, you'll be spending days of messy work processing it, and in the process throwing away two thirds of the raw lichen. I'm not a rabid environmentalist or anything, but I do know that lichen takes many years to grow: anything you pick will not regrow in our lifetimes. Hardly seems justified given its short life expectancy on the layout. Especially when, as Frank says, there are a lot of better alternatives out there now. Just my two bits, like I say I'm no scenery expert - which Frank certainly is.