In a new After Effects tutorial and project, 100% Accurate Reflections Tutorial, Quba Michalski demonstrates "how to build a two-camera rig in order to create perfect 3D reflections. using this technique, you will be able to reflect not only 3D layers, but also lights, particles, as well as any plug-in that obeys the movement of AE's 3D camera.
The setup is trivially easy when using hand-animated cameras, but in the second half of the video I also show you how to create a more complex rig for use with Video Copilot’s Sure Target 2 camera controls."
You can also create reflections with 3rd party AE filters like RGS Warp, Zaxwerks Reflector or the free VC Reflect filter -- and through a variety of other tutorials via the tag reflect. Here's Quba's preview:
Update: Rob Garrott shows Two ways to create a reflective floor in After Effects,