Having both run sound and played with the worship team at church for years, I have to say the sound tech can be the weakest part of the link. But I agree with the previous posters... Not enough weight can be put on a proper sound check and keeping the monitor to mostly vocal.
We use electric drums, so they have to end up in the monitors as well.
The only other thing that HAS to be in the monitor mix in our situation would be the lead rhythm guitar. It's probably different with different types of music.
Here's my general procedure for monitors - We run front (primarily vocal) and rear (most lead vocals):
1. Let everyone get started and warmed up a little. No volume to the mains yet.
2. Mix the front monitors. Lead vocalists, rhythm guitar, drums and harmony singers. Monitor the monitors while you're doing all of this (headphones or some small speakers or something).
3. Mix the rear monitors. Lead vocalists, rhythm guitar and louder drums (so the instruments can really get into it). No harmony singers.
4. Remember that early in the process, vocalist my not be really giving everything and may be still trying to find their voice - so give them time to continue getting warmed up.
5. Do a rough FOH mix to full desired volume (we use a sound level meter to help keep things consistent).
6. Get comments from the band again (you can use your talkback mic). Make adjustments.
7. Go up on stage and stand right next to each person so you can hear exactly what they are hearing from both the monitors and the room. Get more feedback from them, go back to the board and make adjustments.
8. HOPEFULLY, if you've done all that, you are probably getting close on the monitors.
9. After a little more back and forth about the monitors, the FOH is all your's for the rest of the night!
I really think the personal interaction back and forth with the band is critical - force their hand at making comments about the monitor mix. I always let them know that I'm in control of the sound in the room and no matter what they do on stage they are still at my mercy. It's sort of a joke with my teams at church, but they understand I'm pretty serious about it to.
My basic philosophy is that the sound tech is as much part of the band as anyone on the stage. So the sound person has to be as professional (or more so) than the rest of the band...
BTW - Controlling feedback in the monitors is part of this process as well.
But it I was running sound on Friday night after a few beers, I'm sure I would have the exact same problems as the rest of the world!