Many budding live sound engineers make the mistake of viewing the PA system as a means to amplify everything being played on stage, rather than reinforcing those sounds that are having difficulty being heard in the room. If the room is reasonably small, it's a good bet that the drum kit and guitar amplifiers are being heard wherever you stand, so they don't usually need to be reinforced at all. Ideally, getting guitarists to turn down their stage volume, and drummers to play with more sensitivity would enable the live sound engineer more room to work with the tones of these instruments. If the onstage volume is too loud it renders the mixing board controls ineffective. At this point, it's best to keep things as simple as possible and take these instruments out of the mix to avoid muddying the sound and taking up valuable headroom.
Always give yourself plenty of time before a show to conduct a thorough sound check, with all band members present. Have them all play at the volume they'll be playing at show time, to avoid any unpleasant surprises. Bear in mind that when the show starts, and the adrenaline kicks in, there's every chance that everyone will be playing, singing and hitting a little louder and harder, and straining to make themselves heard. You can alleviate this somewhat, by working hard on the on-stage monitor mixes. If you give each player in the band a nice monitor mix of their own, and they're able to hear their own voice or instrument clearly, they'll feel more comfortable and hopefully less likely to turn up their stage volume, which will make your job a little easier.
Vocals are usually the most important part of the mix and need to be heard clearly throughout the listening room. This is no easy task, especially if they're competing with a thunderous drum kit and screaming guitar amps. It's a good idea to use the sub-groups on your mixer to make a sub-mix of all of the vocals. That way, if you need to adjust the level of lead and background vocals, one fader can do it all.
As well as the fader levels, a little EQ can go a long way in helping the vocals be heard more clearly. Cutting lows and giving a little boost to the upper mids can give them a little brightness which helps them cut through the mix without having to keep boosting the volume level.
It goes without saying that drums tend to be very loud on stage and can make mixing a little difficult, since they tend to dominate. If you're mixing in a small room, it's a good idea to first take the time during the sound check to listen to how the drums sound in the room without amplification, and then amplify where necessary. Chances are, all you'll need to do is to mic the kick and snare drum. The snare mic will usually also pick up the hi-hat, and cymbals are so loud that they don't need any reinforcement in small or even medium-sized rooms. For a little extra definition in the kick, try a little boost in the mids and add a little compression to smooth it out.
As far as the bass guitar goes, try not to boost the lows. On the contrary, cut them and add a little midrange to the signal. Try not to boost it in the same frequency range as the kick though, otherwise they'll be competing with each other for space in the mix. The idea is to marry the kick and bass guitar together in a cohesive way without losing the definition of either.
By and large, electric guitars are usually too loud on stage, so unless you can persuade the guitarists on stage to turn down their amps significantly (a notoriously difficult task) you may be better off leaving them out of your mix. They'll usually be heard easily enough throughout the room without extra amplification.
Acoustic guitars are a little tougher as they're quite prone to feedback. A feedback buster (see Resources) for the guitar's sound hole can often remedy this, as can a little experimentation with the EQ control.
Creating a stereo mix in a live situation is largely a fruitless task since unless everyone in the audience hangs out with you behind the mixing desk, few people will actually be in an ideal listening position to hear a balanced stereo mix. Any instruments you have panned to the left will not be heard by patrons on the right side of the room and vice versa, so stick with a mono mix.