This article is from the Miniatures Painting FAQ, by Brenda Klein email@example.com with numerous contributions by others.
Washing comes before drybrushing. Take a shade darker than your
base color and dilute it until it's about the consistency of milk.
Now, brush it across, gently. It'll flow into folds and
crevasses. Makes cloth look real good. Remember, you can always
add wash, so start light and work your way up. Don't be afraid to
wash, then darken and wash again, until you've reached the effect
you like. Wash yellows with yellow-orange or yellow-brown, flesh
with light brown, white with bluish-white or gray. Experiment,
only you can set your style.