What I've been taught: (and we're talking a recreational dive here)
- Dive with buddy that knows how to rescue an unconscious diver, and you have practiced it... often.
- If reg is in, keep it in. If reg is out, leave it out. (Esp if cold water, we know that people can survive after rather long emersions)
- Get in the proper position to keep their head back and therefore airway open (allow expanding air to vent). **Have a qualified instructor teach you this**
- Follow the proper technique of managing their (and your) buoyancy to the surface for a controlled ascent. **Have a qualified instructor teach you this**
- The ascent should be controlled, but no dallying getting them to the surface. You can fix bent (of both you and the victim) but AGEs are a *****.
I highly recommend GUE's Triox class (recreational trimix) where this is a required skill to not only understand theoretically, but practice.