Sound check the monitor mix first with the main speakers off. Provide performers with specialized mixes that highlight their voices or instruments, if your equipment allows. Bring non-essential instrument levels down, rather then simply turning volumes up.
Use in-ear monitor systems, or reduce the distance between monitor speakers and performer’s ears, by raising and angling the speakers as close to the performers as possible.
Bring the overall monitor mix down a little if possible, once the proper balance between voices and instruments has been achieved.
Sound check the main mix, once the monitor mix has been set. Set the optimum overall main speaker level, once the proper balance between voices and instruments has been achieved in the mains.
Try lowering the monitor levels, if necessary to keep the on-stage mix from distorting the main mix.