This article is from the Miniatures Painting FAQ, by Brenda Klein firstname.lastname@example.org with numerous contributions by others.
Two good methods have been presented in rec.games.miniatures. The
first comes from Steven Loren Lane (lanes@spot.Colorado.EDU), and
is used without permission:
% "Well, on top of getting the smallest brushes available, you can
always cut them down to an even smaller size. I have several
brushes that have only a few hairs on them. These are very useful
brushes. I would also recommend for the very fine detail to set
the object up so you can use both hands to hold the paint brush
as still as possible."
And was followed up by Steve Gill:
% "Another useful tool is a 0.13 mm spirograph ink pen, a couple
of splodges of colour in the right place and you can pretty it up
with the pen. I used this technique for 6mm heraldry."
Yet another use for tech pens. They are also very good for shield
devices and clothing patterning.