This article is from the Miniatures Painting FAQ, by Brenda Klein email@example.com with numerous contributions by others.
First off, drybrushing is most effective when used with a colour
a shade or two lighter than the base. White drybrushed over
black primer also makes for a very good painting base. It also
looks good as a stand-alone colour scheme on some figures.
Take your desired colour and an old brush, as drybrushing wears
brushes out and tears them up (the author has had good success in
using cheap watercolour brushes for large drybrushing projects
with acrylic paints, but for smaller areas a better-quality brush
is still necessary). Dip it into the paint until the tip is
saturated, then blot on a paper towel until no paint can be seen
on a dark brush, or a light one looks pretty clean.
Take the brush and gently draw it along the raised parts you want
highlighted. A little paint will stay on the highest edges and
give great depth.
Many painters like to highlight in stages, lightening the shade a
little with each level. This can be either overkill and a pain or
an excellent technique for brightening and preserving detail.
Practice yourself and decide.