Vertical stripes cut the body up into smaller pieces dramatically slimming the area.  Stripes in woven fabrics are more likely to slim an area than those in stretch fabrics due tot the fact your body may cause stripes to curve if the fit is too tight.  The creases in the skirt also fall vertically and are great for slimming the lower body. 

 

Monochromatic outfits are also slimming as they create a continuous vertical line.  The outside silhouette of an outfit can also be vertical which will make you look taller and therefore slimmer. 

 

A fancy central vertical design element can act as a focal point to draw the eyes away from the width of the body.  eg.  A large bustline.