Line and proportion are two topics that aren't talked about as much as they should imho. The line or silhouette of the clothing can make you look taller, thinner, or shorter and stumpier. Think of your favorite piece of clothing or outfit. How does it fit you? How does it make you feel? Now take a closer look. I want you to look at the design lines of the garment. Look at the seams. Which way do they go? Do they visually lengthen you? Do they add curves where you need them and skim/smooth those that need them? Does the garment flow or is it tight?
Some general concepts about lines:
-Vertical lines make you look taller and usually thinner. These lines help the eye visually lengthen as you look up and down. However if strong vertical lines are very far apart they start to look wider.
-Horizontal lines move side to side and visually increase your width. The more contrast there is the wider you will look also (think of a black and white horizontal, striped shirt).
-Diagonals are a bit in between horizontal and vertical. Depending on how much they tip one way or the other they can either be slimming or thickening.
-Curved lines add softness and femininity. You rarely see heavily curved design lines on mens' wear for this reason.
-Straight lines are seen as more formal.Often straight lines are seen in tailored garments. Straight lines can also counteract the natural curves of the body.
Proportion is how pieces make up the whole. For example, how does your jacket and skirt relate to each as part of the whole outfit? Its a type of relationship of your garment(s). Is your outfit pleasing to the eye? Or is it so off balanced that it just looks silly...perhaps a skimpy crop top paired with a full ball gown style skirt would not have a balanced proportion to your eye. With that being said, designers (and nature) often group things in odd numbers. Like 1, 3, or 5 buttons on a blazer or jacket. Odd numbers and odd proportions (1/3 vs 1/2) are often more visually interesting than a completely balanced outfit.
So next time you are putting together an outfit or sketching a new design, take a pause to think about your style lines and the proportion of the garment.