We really struggled with feedback initially, and certainly getting the gain sorted on the mics helped.
Surprisingly though it was the monitor/mic positioning/monitor volumes which proved to be the key. i.e The Monitors were much more important than I thought and once this was sorted feedback is no longer an issue.
Things that helped
1) Matching the stage position of the monitors in accordance with the type of Mics has really helped, Cardiod/Hypercardiod, etc
2) Getting the monitor volume down. Once the band's in full cry there's a surprising amount of sound for FOH on stage, so getting the idea that the monitor is just supplementing the sound that's needed really helps. However, this it the most difficult aspect, as people never say " the Monitors Too loud" .
3) Taken the tack of... "Yes, we can sort out the volume in a minute ....... for now it's just the relative level of each element not the overall volume", if the "balance is right we can adjust as the set goes on IF it's not loud enough. So if someone asks to be louder they may be saying " I can't hear myself over the other vocals (etc.)". So the balance could be achieved by reducing the other elements in the mix.
Still need to remove all the knobs on the monitors to stop the band member deciding to tweak their monitors tone/volume during the gig!!
Or just say "sod it" and concentrate on bass playing.