Ralph has a good point - my experience is that models who have had formal training in ballet are far more graceful that those who have not.
But this is why starting in a workshop setting is so helpful. Someone with experience in recruiting models takes care of that detail, and you get the experience of photographing them and learning what you expect from your models. Then, when the time comes when you want to do it yourself, you have a better idea what to look for.
There are a couple of other reasons why workshops are a good idea for someone who is not experienced. I find that I work in bursts - I get an idea, and with the help of the model we work it for a while. But then I need to step back and reflect on what we have just done while the next idea forms. In a workshop, you tend to work in a tag team with the model - one person working with the model at a time for a few minutes, and then trading off with another person. By contrast, if I am working alone, there will either be periods of inactivity or else the sessions will be shortened considerably. But the bottom line is that they are much less productive for me.
Another consideration is cost. The going rate for figure models is US$15-20 per hour. Spreading this cost over several people in a workshop results in a lower cost per person.
Incidentally, every model that I have photographed (nude) has been a quasi-amateur. That is, they are people who enjoy modeling (and interacting with artists - they view the process of modeling as collaborating with the artists in making art) and who do it as a sideline. Many are artists, and in some of the workshop settings, have also been photographers. None have been represented by agencies. While most have been "young" and "physically attractive", that is not the case for all of them - and frankly, that's not the point. In fact, some of the models have been anything but attractive, but that didn't keep them from being good models.
If a workshop is not possible, something you might try is working with a friend in a less than fully nude situation. The whole point (for me, at least) in photographing the human figure is capturing the way that light interacts with skin, and the way that this interaction can be manipulated by distortions in the skin. You can learn a lot by photographing a model's arm and shoulder. The rest of the model's body can be clothed, but if that arm and shoulder are exposed, and if you study the planes, intersections, and surfaces that are presented as the model moves his arm and shoulder through various positions, you should see things that are visually exciting and potential for good pictures.
After working the arm/shoulder, try the leg - the knee, lower leg and ankles, perhaps in combination with hands. Again, the rest of the model can be clothed - because the only parts that you will photograph are the knee, lower leg, angles, and hands.
Here's an example: http://www.apug.org/gallery/showphot...00&ppuser=2527
Incidentally, this person is not a professional, but rather volunteered to model to support a workshop program.
And don't forget that if all else fails, you can always photograph yourself. Study the work of Jan Saudek - he was his own model in many of his nude studies.