A great rule of thumb is to apply blush to your cheek area according to your face shape.
Heart: Blend blush downward and upward in a heart of v shape with the tips touching the cheekbones.
Oblong: Blend downward at an angle right underneath cheekbone, similar to the oval, however place more further down the face as the oblong shape is longer than the oval face.
Square: While it appears to be on the cheeks our preference is that you place the blush on the cheekbone.
Round: Blend downward at an angle right underneath cheekbone, to the end of the tip of the nose. This makes the face appear thinner and gives more structure to the face.
This video can assist in this process: