DO NOT use in-ear monitor systems with just one earbud in and one out.
The tighter the in-ear monitor fit, the better the bass response will be.
Singers will need a tight fitting in-ear monitor to prevent distracting occlusion (that “inside your head” singing sound).
Bass players and drummers will often need extra lows in their monitors, so they will probably need a different type of in-ear monitor or use larger headphones.
Make sure that the in-ear monitors being used are good quality and work well for the musician or vocalist.
Troubleshoot signal problems and keep a handle on the overall levels going to each monitoring device. Help your team get the best sound that works for them.
Just because you have a personal monitor system doesn’t mean that the sound tech can skip out on helping mix monitors.
Spend time properly training your worship team on the in-ear monitor system so that they know how it works and how they can control their own mix (if that is an option).
Gain affects everything, including the quality of sound in the monitors. This should be done during every soundcheck.