The golden rule to a thinner looking face – the exposed areas of your face should be more to an angular shape, near to diamond shape. That is the summary from various articles and also personal experience.
Some key tricks to make your face look thinner, these are also my usual practice:
1. Contouring your cheeks with a combination of blush and bronzer. Sweep bronzer along your cheekbones and then blend in pink shade blush below the bronzer. This can create contrast which will reduce the roundness of your face.
2. ALWAYS choose a foundation or powder that is 1 or 2 shades darker than your skin. It does not mean you no longer need your normal foundation or powder. Apply your normal foundation and only blend in the darker shades afterwards; darker shades are used for shaping your jaw line to create a more angular effect. But you have to ensure you do not overdo, else your face will appear two tones like trying to cover up some bruises. It takes some time to master the contour technique!
3. The third trick is a hair technique. Try the eyebrow-skimming bangs rather than keeping your hair one length. It will make your face appears narrower.