Two techniques stand out: pushing contrast, and adding textures. You can download free
textures by the tens of thousands -- concrete, parchment, leather, wood, metal, "grunge," etc. You place the texture image on a separate layer (in Photoshop) and either reduce layer opacity or change the blend mode to something like overlay or soft light.
Here's an example using a B&W adjustment layer and then a
grunge layer in overlay mode.