here's my 2 cents, I have done much research on this very topic and I used to have mine on the back wall (parallel to the listeners) and aimed directly at them. They were as close to the ceiling as I could get them and about 6 inches off the adjacent wall. I went to dolby's website and they reccomend puting them about two feet above earlevel and about a foot behind the listener aiming the speakers at each other. The only way I could do this is to move my couch forward about a foot and it made a great difference. They are still high and close to the adjacent wall but they are pointed at each other and the sound is 10 times better, more enveloping. One day I may bring them down but I don't feel like buying another 100' of speaker wire.
The point of this is to mimic the recording soundstage that the soundtracks are mixed in so you hear it as it was intended. Of course I do claim this to be the holy grail, but it works for me.
Natas