In the movies, you see soldiers are very very dirty, and I've tried to copy that onto my figures.
I try to make it look like there's grime, dirt, mud, and excess blood everywhere, especially around the neck area.
I'm very amateur, so my technique might sound very silly, but here it is. Sorry, no pictures.
Light a candle, and arm yourself with some brown/blackish paint and a red permanent marker.
Very quickly put a dot of the red marker on to the desired area and smudge it around with your finger before it dries.
Do the same with some paint, which it would be wiser to apply more thickly.
In the end, brush the spot of the figure across the candle flame and smudge around the black burned stuff with your finger too.
The flame can produce a good five o clock shadow too.
In the end, if you've made any sense of my tutorial, it should look dirty and messy, like he's been sleeping in a ditch for weeks.
Hope you understand me.
Whenever I can get pictures of my Germans on the Pc you will see the effect. It works especially well on helmets and arms.