Complete Idiots Guide to Building a Pyramid by Cheops. Foreward by Rameses III. Sorry. couldn't help myself I think Kaufman's Trading System & Methods has some diagramatic + other info on pyramid structures eg inverted versus normal.
The "Official Turtle Trading Rules" (easily found with a bit of googling) has pyramiding rules built in. The forums at tradingblox.com has extensive discussions of pyramiding. (I am not affiliated with tradingblox, other than as a member of the forum.)
It helps to pyramid into a strongly trending market. Arbitrary rules aren't so important if you can do "that".