A few tips from almost 15 years of playing with IEMs:
The center of your mix is occupied with kick, snare, bass, click, vocals, talkback mics, etc, so for the most clarity you need to clear out as much from the center as possible. The best way to do this is run guitars, keys, overheads, crowd mics, and any backing tracks in stereo and hard pan them all. I’ll even pan click and acoustic if there is one to opposite sides about 11 and 1.
For guitar, if you only have one amp with stereo wet effects your dry will still be centered, so to “fix” this I run two amps dialed in with similar headroom and gain structure hard panned. On top of this, I use the enhancer block in classic mode to really push my guitar out super wide. This allows me to hear my guitar much more easily without just turning myself way up because I’m not competing with the other instruments in the center of my mix. I also high pass on the cab block at 80-100hz, which helps clean up the low end of my IEM mix if I’m getting a pre-fx send from FOH or the monitor console. If FOH is mono, I tell them to just use the left side of my signal in FOH, so there’s no phasing issues.
If you want to pan something that is running stereo (most commonly a second electric guitar), don’t use the pan controls. Leave them hard panned, but just bring down the volume of the opposite side. This applies to FOH as well. This way nothing is getting summed to mono in your mix by panning both channels into the same side, you’re just changing the relative levels of the left and right channels. In the studio world this is referred to as “
balance” as opposed to “panning.”
Do not play with one ear out. When we hear something with both ears we perceive it as +6dB louder, meaning when you take one ear out the tendency is to crank up your IEM receiver to compensate. Playing a full set like this can do some permanent damage to your hearing.
If using crowd mics, use a ducker keyed by something that would indicate when the band is playing, like an aux send of the full band mix. Slow attack/release. This way you don’t hear the crowd when the band is playing but once they stop playing you can hear the crowd in the IEMs. Make sure to pan the crowd mics hard L/R from the perspective of the stage. Most of the time when places have crowd mics there isn't a ducker set up so I just turn them off completely because my preference as a musician is not to have a bunch of pointless mud/reverb/room sound in my ears, but vocalists seem to like them.
Talkback mics are very helpful because they allow the band to communicate without yelling during rehearsal, and live with each other, monitor world, guitar world, and FOH if there’s an issue. Put them on momentary footswitches so you’re not hearing them in the ears constantly and you can’t accidentally leave them on (this has driven me nuts a few times trying to figure out where all the stage noise is coming from in my mix). Radial makes a momentary XLR footswitch that’s very reliable. FOH engineers often have a cheap little speaker at FOH with just the talkback mics so they can hear them since they’re obviously not routed to the mains.
Custom molded IEMs are worth it for the additional isolation, better fit, greater comfort, etc, but there are heavy diminishing marginal returns after like 4 drivers. I played for close to 10 years with the same pair of 64audio v6 and only recently upgraded to A12s. There is a difference but it’s minimal. Only did it because I got them for a ridiculous price.
Sort of goes without saying but if you're hearing the vocals and especially drum mics right off the pres with no compression, EQ, verb, tuning (for vocals), etc, you're going to have a bad time. Singers sing better when singing through compression (they also like reverb on their voice) because they don't get too loud in their ears when they really sing out, and it's also helpful if you're a musician listening to the singer because they don't take over your mix. How we do the lead vocal is take the 2 outs on the wireless vocal mic receiver, run one for FOH and the band through the tuning rig (a UA twin with Console running on the B playback computer). Console is controlled by midi from Ableton on that computer with the keys and tuning strength/timing defined on a song by song basis, tuning turns off during talking parts, etc, so it's all automated. The other line out of the wireless receiver is for the vocalist (the only person who needs to hear untuned vocals) and as a backup for FOH if the tuning rig goes down. Overkill for a cover band with a catalog of hundreds of songs, but our setup is for an artist with the typical max 90 minute set so all the songs are in the ableton session ready to go.