To get the amount of paint on your brush right, remove all the moisture from it after you rinse it out. Dip the brush in the (usually a shade or two lighter than the color you will be drybrushing over) paint, then apply the paint to a piece of scrap paper in a series of strokes until almost nothing is coming off the brush onto the paper. Now, when you move the brush over the mini, the last remant of the paint will automatically stick to the highest parts of the surface. You might drybrush a dragon's scales, for example, in order to highlight the top surface of the scales with a lighter color, while leving the darker color between the scales alone. This gives depth to the look of the dragon's hide.
Painting chainmail black, then drybrushing silver over also it produces a great effect.