Sound on Sound magazine has an excellent article on live sound, which includes some good illustrations of different monitor placements for different types of microphones.

This information is especially important for acoustic bands, as they are likely to have a large number of highly sensitive microphones onstage, and may have a mix of cardioid, hypercardioid, and supercardioid mics in their setup:

Monitors should always be placed as close as possible to facing the ‘dead zone’ of the mic, which in the case of a normal cardioid mic means the foldback speaker should be aimed directly at the back of it. Note, however, that hypercardioid mics have their least sensitive zones around 30 degrees away from their rear-facing axis, and so the foldback speakers should be positioned accordingly.

Read the full article at SoundOnSound.com.

Previous live sound articles:
Handy article on speaker placement
Know Your Mic, Part III
Know your mic, part II
Know your mic, part I
Plugging in