Killer! Dude!
The AUX sends that are PRE-FADER are what you use for the stage monitor mix(es). Some boards (like my beloved A&H's) have selectable pre/post send selection per each (or in pairs) for this, some are dedicated pre OR post. Pre is used for the monitors so that fader adjustments to the subs (FOH) don't mess up the monitor send levels. That bass player who sings bad harmonies and needs loud monitors to key off the others can get that in the monitors and then you can lower him in FOH so the audience doesn't puke. The drummer can hear the bass amp without the bass amp too loud in the FOH, dig? Some boards let you select whether the AUX sends are driven by a balanced XLR +27dBm output for long cable runs back to the stage (i.e, A&H), or by an unbalanced 1/4" +22dBm output to an effects processor at FOH.
And you are correct in using the subs for grouping to go to FOH.
PEACE...

few tips.

NR. 1
MAKE the band turn their amps down and feed them guitars and a little bass through the monitors, so they can hear the guitars everywhere on the stage without having theyr amps to high. When the amps are crancked, it's impossible to mix anything at all! When theyr're set to rather low level, you won't have trouble fitting the vocals and drums in the mix.

NR. 2
Mix the stage/monitor sound before you do the FOH mix. Write down the monitor mix for each band. When the stage sound is good it will be very easy to do the FOH mix and the performance will be much better! believe me, i've tried both, and i always get way better sound when i start with the monitor mix.

NR. 3
When you start the FOH mix, put all your faders down and the master fader just above unity gain. then configure the gain for each channel, and then put the faders up and eq. When you've got a nice sounding and loud FOH, put the master down a little and you will have a great sounding mix that's not to loud. By doing this you will also have ALOT of headroom left on each channel, so you won't run in problems with the kick or vocals disappearing. If you will use the sub groups, you can pretty much leave the channel faders at almost the same mix for the entire show, by just changing the gain for the channels when you switch bands and with the submaster levels.

Anyway.. good luck with your mix :)

Why is this in "Pro Audio Gear" and not in "Live Sound"? It's called a scroll wheel, check it out.

Killer:
We have been possibly assuming some things here. For instance, you may have any number of AUX sends on the board, but how many different monitor mixes will your system support? In other words, you have to have a power amp channel and connected loudspeaker to it for EACH mix you want to set up. You may only have enough gear for a single mix with multiple speakers, or 6 mixes, or something in between. I would recommend trying to have at least 2: a "frontline" (vocal monitors along the front of the stage),and a "backline" (monitors feeding the band) mix. The "frontline"will give the singers a vocal mix with a little bit of the band in the speakers that they can cue off of. The "backline" will give the band a feed of the drums and other instruments, along with a bit of vocals to guide them.
Gnarr is technically correct to say that you need to keep the players' amp volume levels down, but GOOD LUCK!!! I have been in far too many live situations where that was not a valid option, especially with less-than pro players. Be prepared to deal with the politics; don't be a butthead, but try to convince the band that you are on their side to make them sound their best, and that you need the lowered volumes to get the balance right.
And as far as the AUX sends that are POST-fader are concerned, that's for "proper" effects mixing when you use stuff like reverb and echo. Try to have at least an aux dedicated to a digital delay (set to a quick "slap" around 80msec) for thickening vocals and solo instruments, and another to a reverb unit for backup vocals and the like. I could go on, but...back to work... 8-)