Our eyes are easily tricked and you can use that to your advantage when designing your garments. Below are some common optical illusions that you can easily incorporate into your designs.
Here's our first illusion. If you look at this diagram, the top horizontal line of A looks much longer than the top line of B. Your eye is drawn up to the top of A and it gets all the focus. The top lines of both are the exact same length.
This is easily seen when we look at skinny jeans or leggings, shown on the left of the image. Your eyes are automatically drawn up to the hips and they look really wide, even though this model is not heavy.
On the right are standard pants, you can see that the wider leg evens out the body and is more slimming. What's really interesting is that the model wearing the standard pants is much heavier than the leggings model.
Here you can see them side by side. The one wearing the leggings is substantially thinner. Now you can really see how slimming a wider leg can be.
Skinny jeans and leggings are a challenge even for the most perfect figure. They are very trendy, but not necessarily a good wardrobe choice. The back of skinny jeans are particularly dangerous. With the low pockets and all the horizontal lines created with the yoke and stitching, they really add width to the hips and make the legs look like toothpicks.
If you're going to wear them, either cover up the top with a longer shirt or wear boots to make the legs fuller.
Our second illusion is really interesting, here is another set of diagrams. The A figure is fairly balanced but the B figure looks really big. The top lines of both are the same length. Not only does B look a lot heavier at the bottom, the entire figure just looks much bigger.
To illustrate, here's a pair of regular pants and a pair of bell bottoms. You can see the model looks much bigger in the bell bottoms. Also, it looks like she's shorter in the bell bottoms. To extend your height, keep the bottom edge of your garments mid range. Not too narrow or full.
This diagram points out the difference between a vertical and horizontal line of design. You can see that figure B looks much wider than A. In fact, both boxes are the same size and the bands are the same width.
If we transition this to garments, the best example is belts. A belt with high contrast will literally split you in half. The wider the belt, the wider you'll look. In this example, the contrasting belt, collar and cuffs split up her body. Even the buttons, which are vertical, help very little. The garment is all chopped up.
This other figure has a darker overall garment and a contrasting vertical strip down the front. She looks much thinner and taller. The lighter collar also draws your eye up to her face. The look is much more cohesive. If you are very thin, then belts will help you add weight. If not, avoid this kind of contrast.
In our fourth example, we'll look at angling a vertical line to make it appear longer. The angled vertical line in A is the same length as the vertical line in B, but it looks longer.