I'll chime in here with how I do it. But realize that everyone does it a bit different.
I use Paint Shop Pro.
After I've completed the paint, I take a tool that simulates a type of sponge. The color I use is the same as the base. On a military model you might want to use a darker color on seams and such.
I just 'dab' this color over anything that I've added to the base color thus giving the impression that the paints is coming off.
On military a/c when I use a darker color I'll use a tool that simulates a pencil mark, but I'll make it look fuzzy. I'll use this on a 2nd layer of the panel lines and then lay it over the original panel lines thus giving the impression of dirt/oil residue.
I've not yet mastered the are of streaking those stains.
Here are a couple of my reapaints.
Mind you, better painters than me will answer this also but this is just the way that is easiest for me.
Here is a GA aircraft that I fly on VATSIM.
Here is a military aircraft.
