Best Speaker Placement Rules you've used?

I'm a long wall guy, pretty much always have been and i became especially committed to that placement once i got dunlavy's so your milage may vary here. The first order of business is to maximize seperation between the speakers without losing center focus..no toe in and no worries about bass response yet. With the dunlavy's you can't really spread them out too far unless you have a huge room, mine are spread about 10 ft apart.

Next up is dialing in the bass. I read a method somewhere where you start talking with your back against the front wall and start walking slowly into the room. Note where your voice changes and then when it changes again (i tried this as an after thought long after my speakers were dialed in and it ended up with my speakers in about the same place).

Usually, however, my method to dial in bass is to chose the general sopt i want the speakers out from the wall and then start moving the speakers forward and back trying to get the bass to tighten up, minimize boominess and maximize depth. Big steps at first followed by much smaller steps. I've had significant changes with an inch of movement. My guess is that even when using formulas the subtle differences in each room will allow fine tuning benifits to be found if you are patient. I've fine tuning this way over the course of days to get it best for the room. Use references recordings you know well and especially recordings with instruments that you known how they sound live.

Toe in is last and for that it's laser beam towards the ears to start and then tweak it outward to fine tune soundstage and, if needed to soften the highs. Cwlondon's statement about the overrating of toe in may be planer centric as his experience with the monitors seems to support. my speakers totally dissapear.

The long wall set up significantly helps in my current room due to a glass door on one side wall and a big bay window on the other. The long wall gives a lot of distance from the speakers to the first reflection point. some big house plants at those points helped further. My listen couch ended up against the rear wall. As Cwlondon indicates having the seat towards the back wall does help with the bass and here again inches count (what you are doing is manipulating where the speakers and your ears are relative to the room modes but doing it by ear rather than formula). I use pillows behind the head to get rid of rear wall reflections thus making a localized live end/dead end with the dead end at the back. It's pretty much a one head show in the sweet spot with acceptable results outside of it.

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.

From a speaker manufacturer now out of business, I learned to measure the center of the woofer to the floor and multiply that time 1.6. This is the distance to the side or back wall. This then is multiplied by 1.6 and that is the distance to the other. This is NOT likely to be pleasing to others in your life.

I have used the directions on the Cardas and Audio Physic websites with excellent results. AP has changed their directions, and they aren't quite as well written as in the past.