Playing outdoors presents a range of problems, particularly when no suitable amphitheater is available.  Generally speaking, the band will have greater projection if it is seated on some hard surface (such as concrete) which will reflect the sound upwards and away from its source.  Avoid grassy surfaces whenever possible, since they merely absorb the sound.  In such circumstances, bands tend to blast their way through pieces in an effort to be heard and generally tire quickly.  Where possible, it is also a good idea to position the band in such a way that a large wall or building acts as a sort of sounding board to project the sound forwards.