Blush creates an optical illusion, drawing the eye wherever you dust it on. The best place to put it depends on the shape of your face.
To slim a round face: Sweep it along your cheekbones (suck in your cheeks to locate them) and extend the color out to your temples.
To balance a heart-shaped face: Apply blush below the apples (the meatiest parts) of your cheeks and blend out to your hairline.
To soften a long face: Blend blush beneath your apples, but not out to your temples.