How do I ensure that an object stays a certain distance from the camera?

In order to keep an object a certain distance from the camera:

Select the object with the Manipulate component. If you are using a 3D template, it's the WorldObjectController.
Make the Min Distance and Max Distance fields to be the same number under the Manipulate component in the Inspector Panel.

This will represent a ring around the camera. During manipulation, the object will be locked to this ring.