Jackie never wore clothes that were too big or too small for her frame. No matter what she wore, it fit her well.
This is one of the first things to think about when you're shopping. No matter how great the outfit is, it doesn't quite work if it doesn't fit you correctly. Try on clothes before buying them to test them out! Sit down, stand up, walk around the store and raise your arms above your head to see how the clothes work as you move about. Imitate the way you'll be wearing them in your day-to-day life to test their wearability.
If there's a particular piece you love that doesn't hug your figure just right, get a little help from a professional tailor. With a few alterations, you can model that beautiful, sleek Jackie Kennedy look.