How do I get my videos into the "free previews" section?

Free previews appear automatically for subscriber-only content if you have video previews enabled. 

To change this setting click on “All Model Options” on the lower left hand navigation menu. Expand "edit your profile" and change "Video Preview Style" to "Show Video Preview."

You could consider adding a custom preview to your uploads for a more personal touch. You can do this in your “media center.”