Inverse dirtmaps.

Sometimes is very useful generate inverse occlusion bakes to reach an interesting point to start to paint our dirt maps.
Vray dirt material is perfect for this goal, but if you don't works with Vray, is very easy to do the same with Mental Ray and Binary Alchemy Shaders.

Read below.

- You need to install the Binary Alchemy Shaders. Some packages are free and you will have to pay for another ones.
- Apply a "surface shader" to the object and connect a "BA_color_raylenght" to it.
- Put this shader in "Inverted Normal" mode and play with his parameters.

- We reach an inverted "ambient occlusion".

- Use a "blend colors" , "layered shader" o similar to combine this inverted occlusion with a nice bitmap. (use this technique for that).

Sample scene for Maya.
Binary Alchemy Shaders for Maya.

Edit: The most important parameters to play with are "spread" and "far output".