You can also use an old 3D game-engine trick, and do the entire render in one go. Simply make the floor transparent and create mirror-duplicates of all the geometry to go underneath. Spanki did that in at least one of his gallery images.

Hey, who says you can't create reflections in Poser?
Look here! (MPEG video, 758KB)

This was rendered entirely in Poser with no postwork, and before you start singing, "They're cousins, identical cousins," let me tell you that there's only one figure in the scene.

All right, so I cheated. :) I rendered the scene twice. For the first render, I positioned the camera behind the mirror. I fed the rendered video through an editor, flipped it horizontally to create a mirror-image, then imported it into Poser as background footage. I rotated the camera back in front of the mirror and positioned it so that the scene lined up with the mirror-image (the mirror is transparent, so I could see the background behind it). Then I rendered the sequence again, and Sabrina danced with her reflection.

It's a limited technique, but it's faster than raytracing. :)