I find one really important issue is very accurate channel matching, and very similar overall response from each speaker at the listening position. This requires a high degree of room symmetry, at least at the speaker end of the room. Playing pink noise alternately through left and right speakers, the sound should seem identical. I can hardly ever get this to happen but fortunately it doesn't have to be perfect with real music to get a good soundstage. Distance to each ear is also very important.
Getting the tone to a very fine and realistic balance also enhances the sense that the speakers cannot be the source of the sound.