Download our "Setting Up Speakers In A Rectangular Room" PDF here.
The entirety of our thoughts on speaker-placement are contained within this document. If you have further questions, please contact an acoustic consultant. While these companies do not offer free advice, their input is often well worth the cost. A good one in our particular corner of the planet is Acoustic Sciences Corporation. However there are likely similar companies in your area.
Some helpful forums on this topic:
Speaker Placement Calculators
As with the PDF above, these calculators will provide ideal speaker locations,
but do not take into account factors such as asymmetrical rooms, furniture,
window & door location, or spousal approval.
Conventional Monopole Speakers
Dipole / Planar Speakers