A few things you could look at on your shader's attributes are your specularity (lower it a bit) and your diffuse. With diffuse, try and use a map (e.g., a 2D fractal, your own image file, etc.). Try to experiment with diffuse because you can really dirty things up with it. Aside from that, you could just create your own dirty texture in a paint program (or use a tiling image) and planar map it.
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