I'm going to assume that if you're reading this you know the difference between wedge monitors and in ear monitors (IEMs). If you don't, get busy with Google and come back when you get it so we can jump right to the chase. (Wait, but don't you wedge the monitors in your ears and then?...)
IEMs are great, they remove a ton of volume from the stage and leave a lot more space for an engineer to mix in. But there are also a few drawbacks. The first and most important being that standing at the console, your fingers are directly connected to someone's ear canal. While feedback isn't an issue, you can unmute a channel and really blast your players. There is usually some limiting involved so hearing damage might not be an issue, but getting a jolt directly to the cranium isn't the way most people want to start out a gig. Unless they have nerves of steel they're going to get a big adrenaline rush which can lead to the jitters and even physical shaking.
That said, you need to be extra careful when you're mixing for IEMs. My goal when mixing wedges was always to have the feedback eliminated before my musicians took the stage. With IEMs it's to make it through sound check without anyone ripping their buds out and scowling at me. So the pace is a little slower, I check my gain stages a little more carefully, and I gently sneak the levels up until I get the signal from the stage that they're hearing enough.
One thing that can really help, whether you're mixing monitors from the side of the stage or FOH, is to have a pair of buds for yourself. Monitor engineers have utilized a mix wedge for ages, so they can hear more exactly what the performers are hearing on stage. Either way, you connect it to the solo bus of your console so you can listen in on each individual mix. It's easier if all the players are using the same make and model, you can just get the same one. But that may not be practical if you're not always with the same act, or if some members have different pieces. Still though, the best way to do it right is to hear what they're hearing. Even with musicians who are good at helping me put a mix together for them, I sometimes get comments like, "It's just not right, I feel disconnected". That's when I get my cans on and start tweaking as a mixer, not just someone looking for a hand signal for "enough".
Another down side of IEMs is the feeling of disconnectedness that can happen. Some players love this. I have three different piano players that I work with at church and they mostly want piano, high hat and lead vocal, period. But players who want a more balanced mix so they can see how their own playing fits into it can still feel cut off even though they're hearing every input. This is where an ambiance mic or mics come in. The thing that's missing is the sound of the room and the reaction of the crowd. They can be hard to see in stage lighting so it can be hard to tell visually if they're engaged. I won't get in to all the particulars, it's just too wide an area to cover and probably deserves a post of its own.
That's the ups and downs of IEMs, now what about the good ol' wedge? Speaker technology has been progressing steadily over the years and what used to be just a sawed off PA cabinet can now be a custom purposed audio wonder. Features run the gamut and even include subs and array-able pieces. But why use them when IEM technology is so accessible? Besides the issues of RF compatibility in congested air waves, some players just don't like them. Either they've never had a good mix in them, or they can't get past the isolation, or they had a bug crawl in their ear when they were four and get queasy. It doesn't matter what the hangup is, it's time to put a wedge back out on stage and get on with things.
With all the improvements in design you can pick out something that should suit your player well. A bass player needs a box that can deliver the low end. A piano player might need a highly developed bi or tri-amped system. One trick I like to use is put a pair of wedges out that would be too small to do the job by themselves and make them work together. That doesn't mean they double up on signal though, that can lead to comb filtering and cancellation, dead spots that a player can maneuver into. The technique puts instruments in one wedge and voice in the other, or some other breakdown as needed. That way it's a pair of different, clean signals that can both get pretty loud and don't create dead spots or weird phasing effects.
All that really just scratches the surface. Many articles have been written on all aspects of the subject and it's not hard to delve deeper if you want to know more. As always, the purpose of this blog isn't to write white papers or in-depth how to manuals. We're just trying to get ideas flowing and point people in the right direction to get the information they need. So get out there Brethren of the Knob and Fader, look some stuff up, try some new techniques, and if all else fails, hit me up and I'll write some more articles. We're on Facebook and Twitter (links above right) so we're not hard to find.