Every semester I end up explaining this in a really rushed and half-hearted way to a bunch of poor students who are in my animation class. Rather than do that again, I finally wised-up and made a video about it. . .
Somehow my videos seem to get longer and longer, so I split this up after the fact and made two videos (it's like my inner self is telling me to do less videos and more text-y type tutorials).
The first video is the basic stuff for setting up a render pass that will give a nice shiny reflection pass for your scene (stuff like a cell phone or tv in a commercial) and the second chunk of it gives a few ways to customize this concept and make it a more reusable using things like RGB passes for various different reflections, etc.
This is something that is really useful to know how to do in production, especially for stuff like commercials, so hopefully I haven't made it tooooo loooong and boring . . .
Maya/Mental Ray: Rendering a reflection "sheen" pass - part 1 from zeth willie on Vimeo.
Maya/Mental Ray: Rendering a reflection "sheen" pass - part 2 from zeth willie on Vimeo.
BTW, HERE'S the script that I use at the end for pulling stuff out to a render layer with custom color overrides . . . (rt click, save as)