Well, lining where his top and bottom half are is a sort of black canvas-looking cloth, so it would do good to put that on the inside of the bottom half so that when you put on the top half, it looks like it's been tacked on inside. Also, as someone else had mentioned, he's got leather flaps around his 'waistline.' If you possibly can, make the textures fit what part is what. Give a bit of a rubber-like texture to the leather, make what little cloth there is look a touch grainy, etc. If you can effect how outright lustrous something is, take almost all of it off of the leather, and reduce the shine on his outer shell to make it look more like a regular piece of steel than that over-polished brassy look that C-3PO has.
If you want to make him look more like he's been well-used, try your luck with making patchy bits of rust at the edges of the plates and streak it down from the bolts.