The Tips in detail

 "How to apply dirty cracks to objects?"

The key question is: "How close is the object (to the viewer)?". If the geometry is in a close-up, then it's worth taking the trouble to model the detail, say if it's the edge of stone blocks in a wall. For most cases though, the best way is to create "weathered mapping" which requires some work in Photoshop. By using a weathered texture map in conjunction with a corresponding bump map and specular map, you can do quite a lot. On the bump map you might blur the edges a bit to simulate erosion.

An even simpler way is this: Use Photoshop or whatever paint program you use, and whatever you want to stand out, color white; whatever you want to recess, color black and then gradient what's left. (gaussian blur in Photoshop is good for this, especially w/landscapes.) Now you go to the maps rollout, click it, use the bitmap map and select your bitmap and set the preferences however you want. it's quick to render, too, a lot faster than modeling it realistically.

compiled with hints from:
Robert Stava (spitfire@panix.com) and
PyroGX (PyroGX@aol.com)