Jdwek's directions are right in the ballpark. I would simply add that using a solo singer with your preamp set to mono ( if possible ) is a GREAT way to determine the width of the speakers. When you no longer have a central image and are hearing independent left and right signals, your TOO spread out. Move them back inward until they are as far out while still be centralized in terms of imaging.
Once you've got the spacing dialed in, work on the tonal balance and bass definition. By varying the distance of the speakers from the wall behind them, you will be altering where the speakers are located in terms of room nodes. As such, you will be highlighting and minimizing certain frequency ranges. Stop when it sounds the cleanest and clearest while still mainting good body and weight in the bottom end. If you've got monitors on adjustable stands, you will also have to play with their height at this point in time.
Once you've got speaker seperation and bass reinforcement / cancellation dialed in, you'll have to fine tune the sweet spot / soundstage / imaging of the system. This is done by adjusting the "firing angle" of the speakers in regards to your listening position. Toe'ing the speakers in will increase treble detail, widen the sweet spot and make every recording sound "hotter" but can also increase listening fatigue and reduce soundstage width. Running the speakers "flat faced" or "firing forward" can widen the soundstage and reduce brightness, but can also minimize a noticeably "strong" sweet spot. Your looking for a good balance of all of the above. Quite honestly, i think that a good soundstage outweighs a "hot" sweet spot but that is simply my take on the situation. To me, a hot sweet spot is good if you are the ONLY person that will be listening to the system. A good soundstage with solid imaging can be heard in several different seating locations.
Keep in mind that altering the speakers position will also play games with your primary reflection points, so you might have to adjust any room treatments that you have accordingly.
Once you feel comfortable with what you have, start all over again : ( Since your somewhere CLOSE to the final position now, the speakers may need to come together closer / be spread out slightly, moved forward or back, etc... All of these individual adjustments have changed slightly since you first started because one adjustment DOES affect the others.
This is the approach that i've found to work best with any room or speaker combo. All of the "placement formula's" deal with "perfect" rooms in terms of relatively "normal" measurements and speakers with "standard" dispersion patterns. Since many rooms have irregular openings, alcove's, furniture placement, etc... and many speakers do not follow the old "direct front firing dispersion patterns" and are bipolar, dipolar, semi-omnidirectional, etc... the results using placement formulas are less than stellar. Formula's will work for people that are blessed with rooms that were designed by somewhat "intelligent" architects and make use of conventional speakers though. The rest of us are out of luck and simply have to "play" with the system until we can get what we're looking for out of it. Hope this helps somewhat.... Sean
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PS.... You might be surprised at the difference that changes in electronics and cabling can make on speaker placement.
