Dirt/weathering guide?

[Izain]

Can anyone point to a good guide or video for the best dirt/weathering techniques? I can manage chips/scratches/rust/splatter and that sort of thing, but where my results fall short is trying to get good dirt or dust coatings with transparencies. Mine always seem too harsh at the edges, or unnaturally even, etc. Good examples:

Negji

kramenboer

grenage