If you are into church sound, then I hope you have discovered ‘Behind The Mixer’ It’s a great site, and community (mostly via the Facebook group) for church sound guys. They just posted an article today about speaker placement, and if you have the time, read the whole thing here.
Otherwise, let me summarise:
1. Line of sight
i.e. point the speakers at people and avoid things that will obstruct the path of sound from the speaker to them… (unless it is the subs… read the full article for more clarity)
2. Height
I want to say put the speakers as high as you can get them, but people might take that too far… basically if the speakers are too low down, then only the first row or maybe 2 will hear them properly and you will find that you are turning them up too much because you can’t hear from the back…
3. Close to the centre
Put the speakers as close to the centre of the stage as possible, rather than against the side walls. Obviously this will look funny if they are on stands right in front of the worship leader… umm, this is the reason why many people fly a PA over the stage.
The main reason I wanted to write this post was to expand on the final point. Having multiple sources of sound is problematic as this means that a) coverage will be more uneven and b) you can have phase cancellation issues. Unless you are in a large (like +1000 people) venue, then the ideal placement of speakers is shown in the final image on the article: 
High up in the centre, resulting in the majority of people hearing sound from a single source all at the same time.
This is also one of the reasons that I facepalm every time I see a line array in a wide, shallow venue like most modern churches… Read more about point-source vs line-array here.
How are your speakers placed? Talk to me!?