This addresses the implementation of IEMs and using them with various scenarios. I wrote this a couple of years ago, but I think it has some really good information in it:
First you need to decide how you're going to be using it. If your expecting to roll into a venue with a house system and expect them to just plug it in and go, prepare to be disappointed. Unless you advance the idea well ahead of time with the house tech and you're the only band on the bill, the time it takes to create a suitable monitor mix for your IEM system may well cut into your set time. Especially if they're running monitors from FOH. Most are simply not set up to incorporate them or adequately mix them.
If you plan on putting together a integrated IEM system for your group to use, you're now talking about a dedicated monitor mixer (or digital mixer with suitable outputs), a split snake, a mic package, as many individual systems as needed for your band, antenna combiner, paddles and a rack to put it all in. To do it right you need to have everything going through your mixer to get to your IEMs. You can't rely on mic bleed to get it there and if your earphones are fitted properly, there will be an extremely significant reduction of sound getting in from the outside. And don't forget ambient mics to be able to hear your audience. And again, if your planning on rolling into a venue on a multi-band throw and go bill, getting the house tech to integrate your system is likely going to be an uphill battle without proper advancing.
Having said all that, if you're running your own board the majority of the time and just want to add IEMs for you and you mic everything, it can easily be done. My system preference, after several trial and error runs, is the Sennheiser EW 300 Series. There are cheaper systems out there such as the Galaxy Audio or Carvin which work but are limited in features, sound quality or other factors. Other reliable brands such as Shure, Audio Technica, etc. offer good options as well. But remember, this is how you will hear everything. Get the best you can afford and be sure to check the frequencies it uses against what's open your area. Sennheiser has a chart online to help you select the right frequency band for your use. I use the A Band as it seems to have enough open frequencies in the western United States.
The actual in ear earphone phones are about as personal of a choice as it gets. Again, this is not where you want to cheap out. I use a set of custom molded Westone ES-10 earphones and some old Shure E2s for back up. I also keep spare cables for the Westones on hand. The earphones that come with a new unit are generally barely adequate because most people have a personal preference anyway and are most likely going to upgrade. Custom molds are worth every penny. It is a mold of your ear so they seal very well, block outside sound and have better low end response.
If your going to monitor in stereo you'll be using two aux sends on your board. This is not an issue on most digital boards as they offer more output options but will eat up aux mixes quickly on analogue mixers that aren't dedicated monitor mixers. Many will tell you that you don't need to run stereo monitors. I tend to disagree because I have two ears and hear in stereo. Stereo also allows you to place instruments within that stereo field and that separation makes for a much more pleasant mix to listen to.
If your a drummer or keyboard player there are hardwired options that will save some money compared to wireless but your choice of earphones would be the same as above.
Now the don'ts, or at least shouldn'ts. Do not use only one side of the earphones at a time. You'll end up cranking the in ears to keep up with the open side and possibly damaging your hearing. Being able to monitor yourself at reasonable levels was one of the benefits of IEM. If you are researching this on the internet you will undoubtedly come across some version of the "poor man's in ear monitor" at some point. This is usually a cheap POS headphone amp into some cheap earphones. Don't. Just don't. A true IEM system will have built in limiters that act very quickly to clamp down the output when needed. Without it, when your singer decides to jam his mic into the horn of his wedge or your guitarist decides that 11 isn't loud enough, you basically get an ice pick to your ear drums. Hearing damage is often irreversible. It maybe too late to stop the damage you've done so far, but it's never too late to keep it from getting worse.
That should give you some things to think about. There's lots more to know and lots of people more than willing to tell you their cheap it's-good-enough stories. Find a professional website forum and do a search there. Read every thread you can. Remember, you're not breaking new ground here. Most of your questions have probably been answered but don't be afraid to ask questions if you don't understand something. If you are a singer be sure to read up on the occlusion effect. Stick your fingers in your ears and sing and listen to the sound of your voice. That's the occlusion effect and it becomes a big factor in what you hear with IEMs. Good luck!
