While I havn't been in this buisness as long as an artist such as Mr. Crumley, I have used multiple techniques, and it depends on the surface you are mounting to as much as anything.

When mounting to a wood wall,I've had really good luck with HDU in simply fastening through the face of the sign with 2-3" wood screws, and color matching the heads. But most of these signs are wood grain textured in a blasted type format ,and the screw heads almost disapear .

I have recently started milling slots in the back side. I then weld a simple "H" shaped frame together in 1/2" tube steel for ridgidity.
The wall has a simple frame that the sign frames slides into(telescopically) and the two are held together with set screws.

If you are interested I can e mail a drawing. It's much simpler than it sounds . And you can acheive a cool dimensional stand off from the wall as well ......if you and the customer choose .