Ahhh yes in ear monitors!
Here's my 2 cents of things I've learnt while mixing monitors over the last few years... Hopefully I don't talk too much gibberish!
Whenever I mix IEM I have the luxury of stage ambience mics, usually a LDC on each side of stage (or sometimes hung in the roof.....) that gets hard-panned in all the mixes and occasionally an additional stereo audience mic.
I find that it really helps 'create a space' in the performers' head by means of connecting with the audience reaction, as has been previously mentioned, and also so that the mixes aren't bone-dry.
The easiest way to get some ambience mics is to run an SM81 (or equivalent) on a stand either side of stage and either run it into the drop boxes or straight into your console. If you don't have SDCs use anything, it would sound better than nothing at all and wishing you'd connected something up. You can always turn it off!
A healthy dose of ambience in a drummers' mix can really help to get that sense of the room sound but in a controlled way. Riding the ambience mics during certain songs/audience participation moments greatly adds to the vibe too. I have my IEM mixes pretty live sounding and get some nice comments on how it sounds, some people like it excessively live, others don't like any ambience, it's all personal preference...
If you're using click simply DI it into the monitor console and make sure everyone has their desired level, some people prefer it to be panned any which way in the stereo image. If you can get the click line up and running early on it'll help the drummer during drum check.
I pan mainly electrics, acoustics and vocalists as they appear on stage from the musicians' perspective. sometimes keys, depending on what they're playing and where they're located. if two people are singing, I'll pan each singer in the center
for their own mix but off center
in everyone elses mix, depending on which side they're standing.
Of course a talk to stage mic is needed... I take a cheap switched mic around with me and leave desk channel open to all the IEM sends during sound check.
Using a reverb unit helps for lead vocals, mainly to support the ambience mics, where I try to run a separate FX send for each lead singer. When using digital consoles it's pretty easy to route FX around everywhere and use extra verbs for snare + the whole kit.
As for digital consoles in general, I make a template scene during prep/line checks then save as another scene when sound check starts and then save another scene again after sound check. Any support acts/acoustic sets/items get their own scene too.
I don't compress too much when mixing monitors as the band needs to hear their dynamic changes, somewhere around 3-6db on kicks & snares, up to 6db on dynamic singers and I might compress a few other channels. Seems to work fine.
I don't EQ too much on bass, guitars and keys as I'd prefer to get good tone from the source however I do a few dips on bass & keys to make it sit better in the mix... this is all the usual stuff... guitarists HATE it if you EQ their guitar.
Using HPFs helps a LOT for IEM mixing, I use them on every channel except kick & bass.
Please make sure you're using good quality IEM drivers and not cheap ipod earphones... they sound absolutely terrible and the band won't stop complaining. I've had Ultimate Ear molds for over a year now and I still believe it's one of the best things I've bought.
Always try to run stereo ears as much as possible, I wouldn't waste my time mixing mono ears... trying to 'create a space' in mono won't be easy although it can be done if it's a small band, the arrangements stack well and with a bit of picky EQing; similar to mixing an album in mono
I agree the whole 'one ear' thing is ridiculous... if they're taking an ear out then there's something wrong with their mix, sometimes it's because they can't hear themselves, anything to pitch off or the crowd.
well that's a few things that I've learnt, hopefully it helps a few people out. see how you go!