This technique is oftern overlooked yet it is simple, easy and effective. Dry brushing is used to add whats know as highlights to your model. Highlights are lighting on the highest parts of the model. These will be in direct constrast with the areas in shadow. Take a look at fig 1. this will demonstrate.
It is know as dry brushing due to the fact your brush should be almost dry when applying the paint. Start by mixing the colour of your highlight, normally its a couple of shades lighter than your base coat. Once you are happy with your colour, clean, wash and dry your brush untill its bone dry. Now pick up a small amount of your mixed colour with the tip of the brush and proceed to wipe all the paint off on a piece of tissue untill barely any paint is coming off the brush. Your now ready to begin drybrushing.
The art of dry brushing is not to apply too much paint, the golden rule applys here as ever; less is more; one can always add paint later but removing excess paint is hard. Your only going to be painting the tops of the raised areas and not the recess, this is golden rule number two. Never highlight an area that is not prutruding.
Now grasp your brush and hold the tip at a right angle to the raised surface moving the brush in a left to right, right to left motion. Because the brush has very little paint on it you will need to do this several times but you will begin to see very small droplets of the lighter colour being deposited on the surface. Continue this process untill the raised surface is virtually covered, although try to aim for a rough speackling more than blankey cover.
As with all techniques this does take some pratice. If you find that too much paint is being applied then your brush isnt dry enough, if your drybrushing doesnt add enough paint it is too dry. However once you have mastered this technique its a very quick process and as many a modeller willl tell you, once of the most exciting as it really brings the model to life. Drybrushing works best when combined with good shadows and lowlights acheived through washs.