I started with a plan.  By spending a few minutes right up front, I was able to quickly determine where to install the speakers. When I purchased the speakers, I knew I would put 5 in each wing of the building, plus 2 in the lobby but I didn't know where they were going to go.

When I went to the space to install them, I didn't just start.  I made sure I walked the space.  I put a marker below every ceiling tile I figured would be where the speakers would be best in.

The person who knew the layout of the space helped me better understand where the seating was going.  Without this insight, I may have had a slightly unbalanced sound.  With that meeting done, I knew where the center of the aisle between cubicles and desks were.  With my new markers in place, I walked a final time.

This last time really was just to imagine the sound coming out of the speakers.  I wanted to visualize the office with furniture and walk as if I were in the finished space. 

Intuitively, I just knew I had the right distance between ceiling tiles.  Well, how many ceiling tiles between each speaker you ask?  You don't really need an answer to this.  But, just for kicks, it was 6 in most areas.  It's not space between that is the consideration, it's evenness.  When the final install is done, you don't want peaks and valleys in the volume as you transverse the space.

Make sure to walk the space imagining the volume of each speaker rolling off where the next would start to pick up.  I figure every speaker has a circle around it where the best sound is.  Figure it's 10' to each side of the speaker or something like that.  If you are worried, pick up a few more speakers.

In the end, you have to know what the purpose of the system is.  If you want non-distracting background music, stay away from putting speakers right above desks, meeting areas and reception desks.  Instead, have the speakers trail somewhere in the middle of these areas.

You want everyone to be able to talk comfortably and be on the phone without too much sound right on top of them.  If you have announcements or something that is very infrequent, you'll have to gauge that for yourself.  Too few speakers and announcements will be unheard by some.  To many and you've just spent a lot of install time and extra expense (albeit not very much per speaker) and may need a bigger amp or additional amps.

A note for retail stores - my strategy would vary slightly for retail stores.  I would consider much punchier high powered and higher-quality speakers and just use less of them.  Some of the best stores I have been to have really nice clear overhead sound and they play it at a nice volume. This is something ceiling speakers are not really that good at. That's more of a consumer stereo system or an actual performance PA.  In a retail setting where you want to pump the music, you should look at another option than the cheaper ceiling speakers.