I seem to remember the magazine "Sound on Sound" recommending turning everybody else down instead of "upping" one instrument.
Otherwise everything just gets louder and louder and louder and....OOOOwwww
Set the input gains on your mixing desk to just flash the clipping Led or to just enter the red on the vu meter for each and every instrument individually with no-one else playing. This in itself will set every instrument to the best setting. Use the faders to adjust the front of house balance. For foldback, get the clarinet to a good level in the monitors and then balance everybody else around that, and keep it all DOWN to a comfortable level.

You have a duty of care to your musicians(and punters), that by law stops you causing them hearing problems.

Chris (Amicus AEEU Safety Rep)