Keep in mind that horizontal lines move across the figure and shorten it, and when you shorten a figure, you broaden it. Vertical lines move up the length of the figure and lengthen it. When you lengthen a figure, you slenderize it. Diagonal lines are slimming because they keep the eye moving. The longer the line the better.
Curved lines enlarge a figure, adding pounds and inches.
The eye travels in the direction of a line. Lead the eye to details which flatter your figure, toward your best features and away from figure imperfections or challenges. Remember to take into consideration the effects of details added to a basic garment. For example, a contrasting bias binding added to the neckline of a basic dress draws the eye upward toward the face, extending an existing vertical line and drawing the eye away from the figure. On the other hand, contrasting bias bindings at the neckline and skirt hems would act together to create horizontal lines shortening and broadening the body.