Layered Textures/Materials

So I have a character, and I want to lay a texture over his body. Let's say, an undershirt texture over his skin texture. I've heard that listing multiple materials would do it, but they simply blend together when I do it, instead of laying one on top of the other.

Quite simply, what is the most flexible way to allow textures to be displayed over other textures?

PS: I've heard something about submeshes, but I can't seem to find any proper information on what they are in relation to this problem. Would submeshes help? If so, would they be something I have to implement for every character model, or is it something I can automate?