Have you ever gotten dressed, look in the mirror, and realized that something doesn't look right, but you can't put your finger on it? It doesn't matter if you spent $10 or $1000 on the outfit- if your proportions are off, it won't look right. Take a look at this:
These two center circles are the exact same size. But by increasing or decreasing the proportion of the circles around them, one center circle looks larger than the other. The same applies to your body. Knowing how to use these proportions can help you play down a large bust, give some volume and lift to your rear end, give you a leaner midsection, and the list goes on and on and on.
|short (L) long (R)|
What does this look like in real world application? Just for example, let's say you're long waisted* (see definition below). To look leggier, wear dresses and tops that are cut to accentuate above your natural waist line (ie- empire waists). You can also wear blouses tucked into higher waisted pants. Visually decreasing your torso will make your legs look longer by comparison. You can clearly see this in the picture below.
The opposite it true for people with short waists. Does this make sense? And it all has to deal with visual proportions.
*"To determine if you're long-waisted or short-waisted, measure yourself from the top of your head to your natural waist, and then from your natural waist to the floor (you'll probably need to enlist help to get an accurate measurement). If you're longer from head to waist than from waist to floor, you're long-waisted, and if the opposite is true, then you're short-waisted." (http://www.stylephyle.com/experts/long.html)
Image(s) courtesy of InsideOut Style and dummies.com