Tune out unused frequencies. For example, your kick drum will put out only low-frequency sound. For its channel, turn off the high- and mid-frequency knobs. Eliminating unused frequencies usually eliminates feedback as well.
Place mid-frequency instruments in slightly different spaces on the sound spectrum. For example, if you have a lead guitar and a rhythm guitar, both will put out most of their sound in the mid-range frequencies. If the channel equalizer settings for both are identical, it will be difficult to distinguish one from the other. Turn up the low-frequency knob on the rhythm guitar to give it a distinct place in the spectrum - this will also make the lead guitar sound clearer by freeing up space on the sound spectrum.
Tweak, tweak, tweak. Once each channel is free from unused frequencies and each instrument has a unique space on the spectrum, make more adjustments until you have eliminated all feedback and the band sounds perfect.