Your first line of defense is placement of the monitor. If you've got a wedge in front of you, find a new place for it, and keep in mind both the pickup pattern of the mic and the way the guitar top might be reflecting sound back into the mic.
Often the best place for a guitar monitor is behind the player, so the player's back acts as a shield between the monitor sound and the mic. Sometimes side fills will work better than wedges in this situation. Of course a lot depends also on the room you're in and the sort of stage you're on. Are these problems you're having at particular gigs or is it in a rehearsal space?
Often in the case of moderate volume acoustic music, finding the right position for the monitor and a little more attention to dynamics from the band will be all you need.
The next line of defense is EQ. Aaron's recommendation of a 31 band EQ on the monitor signal is a standard way of dealing with this. Another way is to get a preamp with a notch filter on it--and a phase switch can also often help a great deal. There are also some pretty good electronic feedback destroyers out there that will automatically notch out any frequencies that are causing feedback. Sabine, for example, makes one that works well when used on one problematic signal chain (it can be put in the channel insert, for example, of your guitar channel at the mixer). Make sure that you are also engaging a HPF on the guitar signal somewhere in the chain (at 80 Hz at the very least, but 100 Hz or even as high as 200 Hz or so can make a big difference in low-end feedback, and it will also help the guitar stand out in the mix). For boosting, instead of the clean boost, try an EQ pedal for boosting. This way you can only boost the frequencies you want for the lead (mostly in the mid and upper ranges--it's often a boost in the bass that causes feedback for acoustic guitars). An EQ pedal like the Boss GE-7 would let you set a boost level at only the frequencies you need and to leave the bass frequencies where they are--or you can even attenuate them.
Try and keep effects like reverb out of the monitor mix, too.
If none of this works, then you'll need to rethink the use of an external mic (or at least the mic only). Using a pickup for part of your signal--especially for the feedback prone part of it--can make a big difference, and you might still be able to use the mic to give the sound some natural "air," as they say. There's lots of great information on this forum from people who use dual source systems.
Simplest solution, of course, is to get either an undersaddle or a magnetic pickup. These are feedback resistant. it's just a matter of whether or not you feel you can get a sound you like out of them. Digital imaging systems like the Fishman Aura are another way to use a pickup and get something like a mic'd sound out of it.
Louis
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