A good way to make things appear less repeated is to use masks to belend between different versions of the same texture or to overlay dirt maps or other textures on top.
You can use the mix8 layered shader (if your using mental ray)
or mib_color_mix.. however if your not using mental ray shaders maya maya layered texture works fine.
Here is a quick example:
Using a fractal texture which is layered over the orignal grid using another texture as the alpha will give you some variation over the entire surface. In this case I also blended between this result and the original using a ramp so for example if you only want the bottom of your wall to be dirty you could achive an effect this way.
I know its rough but hope you get the picture. There are many ways to blend and effect textures, color gain is really useful, to stack up results, ramps are nice to control effects over a surface and the math nodes and surf luminance nodes can be used to drive textures based on lighting contributions etc.
FX supervisor - double negative