Assuming a reasonably symmetrical room, try putting the speakers 29% of the way across and lengthwise (for regular dynamic speakers; not sure about other technologies) and sitting in the corresponding 29% point lengthwise, in the middle widthwise, should yield truly excellent results.
This tip is from Chris Klein at ASC, and it certainly worked wonders in my listening rooms. Apparently several typical room modes are avoided with this approach. There's a remarkable improvement in articulation, clarity, transparency, and flatness of response.
Some speaker manufacturers (Usher, for example) recommend a similar strategy, using 20% or 33%, perhaps on the assumption that calculating 29% would be too challenging

. These work OK, but 29% was radically better in my space, which is pretty much a 2:1 aspect ratio with the speakers pointing down the long dimension. Depth of soundstaging, space around instruments, realistic portrayal of instruments and voices is impressive.
On the new Reference Recordings hi-rez Britten files, the ranks of the violins, for example, are clearer "visible", spread out across the soundstage.