Best Speaker Placement Rules you've used?

Diagonally across a room corner, so the central listening position is located at 1/3 room length and width.
ie: for 12x15 room, listener is 4' from one side wall and 5' from the back wall.

I've done some basic frequency response measurements with various arrangements, and this one produces by far the smoothest and most extended bass response.
It also tends to reduce the impact of early sidewall reflections, since the sidewalls "splay outward" like a horn, instead of being perpendicular to the speaker baffles.