Place the microphone close to the source: the farther the microphone is away from the source the more the chances are that the 'open' microphone will pick up the stage levels.

Use microphones with a cardioid pick-up pattern: this pattern rejects any rear protruding signals effectively or even if you are using other unidirectional patterns, ensure the wedge placement isn't in the path of its rear pickup (see pic below)

Keep the monitor mix send levels to the monitors under control, this is a challenging one because the moment you reduce the monitor levels the musician begins to ask for more and if you keep giving more you are sure to induce feedback, and then all eyes turn toward you.

If you don't have a graphic equalizer for each wedge, then with the help of a parametric equalizer on the FOH mixer (preferably in the Master aux send) you can also ring out any possible feedback. You will have to be alert in listening to the offending frequency or the frequency which generates the ringing. Place a microphone in front of the wedge and then slowly increase the gain of that particular microphone as you increase the gain for the mic, you will at some point begin to hear a ringing like feature, it could lie anywhere in the region of say 1 to 5 k, if the wedge on stage has a 9 band graphic equalizer you can use that to cut the offending frequency or else your last option is to cut gently, the offending frequency on that microphone (mixing console) but bear in mind that if you cut frequencies in that region you can affect the vocalist's intelligibility, or in other words, you can't hear important syllables clearly.