As an Image Consultant, one thing I often hear from clients is, "How can I dress thinner"? Well, the best way is actually to dress according to your body shape. But if you aren't sure of that, there are some general ways you can dress to appear taller and/or thinner.
1) Dressing head to toe in one colour (though this isn't my favorite look), and dressing in tones of one colour (which I prefer). For example, dark charcoal jeans with a metallic silver tank, and maybe a light grey cardigan.
2) Wearing vertical lines. In the form of seaming, stripes, creases, and centre front buttons. FYI...the closer the lines, the slimmer the look.
3) Wear non chunky shoes with a medium to high heel.