I actually know exactly what you're talking about. I've known a number of artists who could draw a flawless human (face or full body) on a whim while they were sitting next to me. Where as I need to basically copy down an image I see and even then the results seem frustratingly distorted. I've come to discover, however, that the people who can do that are either extremely rare, or highly practiced and I just didn't know it. Think of it this way; when an artist wants to draw a human character from their imagination, rarely are they going to create a fully original concept. The hair might be from a character they saw in a cartoon, and the nose is from a favorite actor. Then the clothing is from a video game they played. The things we create are composites of everything we know; so nothing is 100% out of our imaginations. It is the culmination of all of our knowledge that creates something new.
Practice does make perfect, but I would also recommend how-to-draw people books and one of those little, pose-able wooden men. Start by just copying down the images, not tracing, but using your eyes and drawing it free hand. The more you do this, the more you'll get a feel for how to draw a human. The pose-able man gives you an idea how how the human body looks in various positions so the next time you sit down to draw, you can more accurately remember perspective.
There is no quick way to draw perfect people from one's imagination, but it is certainly a skill that can be learned. I can't say I'm very good at it myself, but that's because I discovered DAZ Studio, and the need to draw my own characters diminished.