Ahh, the eternal question...
Ok, if you have dipoles/bipoles then I don't have much to say since I don't have either. If memory serves, bipoles/dipoles should be placed immediately to each side of the primary listening position so you're in the "null".
If you have direct radiating type surrounds then read on...
Location
Dolby recommends that rear speakers be located between 90 degrees (directly to the side) and 110 degrees in relation to the primary listening position. As shown here:
I have a combo living/dining room like yours and I found my surrounds too distracting when they were placed at 90 degree angles. I would first try them at 110 degrees or more. I'm limited with my options because I have a window at the 110 degree location. So my only option was to put them on the other side of my window, at approx. 135 degrees. I found this location to be far superior in terms of providing the diffuse sound you want from surround speakers. My surrounds are mounted directly to the wall so their front baffles are facing each other. Here's my room layout (disregard the 110 degree notation, it's more like 135 degrees):
Tilt or Toe angle
I encourage you to experiment with toe angles because you might find you need to angle them toward your listening position the farther back in the room they are. Just put on a DVD with good surround effects and compare. I recommend The Haunting DTS. Crappy movie but great rear sound effects. The goal is that you don't want to be able to easily discern where the sound is coming from, i.e. diffuse.
Height
Rule of thumb is 2 feet or so above your ears at the primary listening position. Art Noxon, a room acoustic expert, has authored a good article about surround speaker height. You can read it for yourself here:
http://www.asc-home-theater.com/htvol3.htm
Here's the nitty gritty from his article:
"...minimum position for stimulation of the second and third resonances lays 20 percent from one end of the dimension (ceiling height). This means the best, anti-resonant location will be a distance down from the ceiling that measures about 20 percent of eight feet or 1.6 feet (19 1/4 inches) down from the ceiling or up from the floor.
If you have brackets that allow you to adjust downward tilt you can try angling them down into the center of the room. But you may find this detracts from the diffuse effect you're looking for.
Good luck and enjoy it!
PS - You may find you get an even more immersive (and less distracting) effect if you run your surrounds 1 or 2 decibels below your front mains.