It doesn't matter how good a monitor engineer you are. If you have severe halitosis you will not be asked back.
Things that I learned/re-learned from watching Mixing Monitors from FOH on Soulsound.
Although each physical microphone input on a mixing console can only have one headamp gain setting, each input channel on a digital desk has in input attenuator.
On a digital console, duplicate all inputs and setup a completely separate mix for the stage, even in small venues!
Listen to monitors in pairs that are near each other to verify polarity.
If you are going to double-up monitors on a vocal mic, make sure to aim them using the HF driver, not the center of the box.
Place the stage monitor as near to the musician's head as possible.
If you are using the sidefills as vocal reinforcement for a singer standing center stage, everyone stage left and right is going to get blasted. Using a short stack (