Despite the time we spent above on equalization, most problems of sound being difficult to hear are really volume-related. Start by getting the overall volume right before you spend a whole lot of time playing with EQ finepoints, and follow the order of this section, or you'll be wasting time on things that may actually be counterproductive.Check the Mix throughout the Hall
You can't mix well from right in front of the stage:
All you can tell from right in front of a speaker is that it is working/not working, or maybe that a certain instrument is not coming out. Get out into the hall to hear what the crowd is hearing, or you'll really mess up the mix!
Read that again. Slowly. Twice.
Part of the key, especially in the acoustic nightmares that house most events, is the following rule: when in doubt, cut levels rather than boost them.
That is, if instrument A is indistinct, think whether someone else may be too loud before boosting A. I've found it often helps to just cut the loudest thing on stage, and then maybe to boost the overall volume.
If you reverse this rule and boost all the time, pretty soon all the channels will be maxed out and the overall volume will be incredible - it's like a bad haircut joke, where you take some off one side, oops, no now this side, er uh...
Your goal is to get the level to just the point where it has what I call 'presence'. This means you can understand the caller easily without effort, and distinguish every note that each instrument plays. It often sounds surprisingly good if you just get this part right. Try setting everyone to the same level (except for very quiet or very loud instruments) and just varying the overall volume, and you'll get surprisingly close to the best mix, without EQ or other futzing around.Adjusting as the Dance Grows
As people enter the dance, the number of bodies absorbing sound and generating their own noise greatly affects the overall sound in the room. You need to adjust for this, often just by raising the level slightly. Reduce when the crowd thins.
What you don't want is for multiple people to be twiddling dials without communicating. This is a sure recipe for disaster in the hall, and will drive the band crazy. Make one person responsible for all changes.
While a second set of hands is bad news, a second set of eyes and ears is great to make sure you're not overlooking anything in a complex situation.Calibrating your ear
The way you listen is crucial. Sound is information - what do people want to hear? What do the band and caller want to hear? What would you want to hear if you couldn't hear anything else? If it sounds good loud, will your ears get 'tired' quickly? How do the same people sound without a system?
Think about these things while mixing, but also as you listen to recordings. Know what the music sounds like at its best, and have a target to shoot for.Working with the Band
One joke has it that the difference between a sound person and a toilet seat is that the toilet seat only has to deal with one asshole at a time. Unfortunately, like a lot of humor, this one has occasional justification in truth.
But if you're dealing with a well-known touring band, they haven't gotten where they are without knowing sound - probably at least as well as you do, plus they're experienced with their own needs and the way they want to sound. You may know your hall and system better, but listen to them anyway!
If the band isn't as experienced, then a lot of your job consists of keeping them relaxed while you figure it out for yourself. The key here is not to let on that it sounds awful, or they'll freak out, and then it really will sound awful. Be reassuring, and work carefully and deliberately.
A caution: endless tweaking can drive the band crazy - watch that you're not being compulsive here, and dance! And regardless of experience level, tell them that it sounds great when it does! The band always likes reinforcement that yes, it really is working out front.
Easy-going humor can be a big help. Make sure it's not sarcasm, but honest, simple, harmless humor. Anything else can come across wrong in a noisy stage situation. But if you do it right, it relaxes everyone and lets them and you do the best job possible. When in doubt, a plain smile is always in style.