Strong contrast pops. It draws the viewer's eye and seems to come forward in space. If you don't control it, you'll have distant features coming forward, near features falling back, resulting in distortions in your drawing. You don't have to be a slave to nature - sometimes you have to modify what you see to make it work on paper.
Help establish three-dimensional form by making closer parts of the drawing more high contrast and crisp, and distant areas softer and lower contrast. This is especially important when drawing faces at three-quarters view. The side of the face that is furthest from you needs to seem further away in space. If you have a dark shadow against white paper, or light skin against dark hair, that side of the face can seem to come forward, spoiling the three-dimensional effect you are trying to get. Using more gradual changes of tone, recucing the contrast a little, can reduce this problem. Make the closer areas of the face come forward by carefully lighting the subject so that you can use stronger shadows and lines to model the nose and the side of the face that is closer to you.
Imagine a landscape: that tree in the middle distance might look quite dark to you, but if it contrasts strongly against a nearby building, the contrast 'pop' will bring it forward. You don't have to draw it faintly - we aren't talking atmospheric perspective here - but just knock the contrast back a little. Don't make your darks quite as dark, and perhaps make the side of the building a little less white. Gradually lighten some of the foliage towards the building and add a soft shadow to make the change more gradual. Make closer objects come forward by drawing them more crisply, making the highlights brighter and the shadows darker.
