Very nice character, I want to see more!
Just adding a bump to the costume surface won’t make it stand out from the skin because the costume surface (incl. the bump) has no relation to the skin surface.
If you wouldn’t want to add geometry you could try with some weightmap+gradient powa! Just create weightmap which outlines the areas where the skin and costume meet (use David Ikedas Select Outline) and then add a gradient in the bumpchannel of the costume which uses the weightmap. (Make it so it bumps (value 100%) only where the weightmap is 100%.) This will give you highlights and shadows on the fringe of the costume. To enhance the effect with a slight shadow cast on the skin use the same weightmap in the skin surface’s diffuse channel.
(an example using slightly modified head by Taron)
You might have to tweak the flow of polygons a bit to get rid of artifacts, and you should try to do away with large differences in polygons density around outline if you want an even looking thinkness to the costume. Although the results above look great already I think your character might acctually be more ideal for this than Tarons head.