Apart from click and room mics, which people have already mentioned, one thing I do often on mixes is create parallel IEM mixes for vocal lines to fight occlusion. So, vocalists each have a channel for their vocal mix, just for them to listen to, completely separate from the front-of-house send, and even separate from the vocal mix anyone else gets of their vocals. This allows me to EQ the vocal mix of their voice so that what they hear of themselves singing is clearer and not simply louder. Any time a singer can't hear themselves in their IEM mix, it's almost always not because they aren't loud enough, but because they are fighting occlusion.
This is a good video about it: