What to do about that unwanted beast feedback! The reasons for getting feedback in an audio signal can be many; environment, surfaces, equipment, placement, and even weather conditions. If you're getting feedback there's a couple of tricks you can try for solving the problem, and they aren't necessarily that complicated. In fact, the simple ones make a lot of sense.
Outdoors you will most likely have less problems with feedback as there are usually less reflective surfaces than indoors. Regardless of indoors or outdoors the following suggestions should help you improve things.
First, speaker placement. Give your speakers good separation and aim both at a central point two thirds of the way to the back of your location. Indoors, each individual room has its own acoustic properties and resonant frequencies. So, you can't rely on the same placement solution to each new room, you'll have to tweak a little.
The simplest thought on dealing with feedback is to 'break the loop'. In other words, move your microphone to a place where it is not getting any of the unwanted signals which are causing feedback.
EQ is may also assist, but in doing so you will be affecting the entire output signal and losing some of the sound quality. Find which frequency your feedback is occuring, chose the smallest bandwidth possible and eliminate (or reduce) that frequency. There is a slight problem that whoever is monitoring you sound will have to keep an eye, er, ear on. As mentioned earlier the exact feedback frequency can shift due to conditions such as temperature and humidity. An empty room that you do a soundcheck in may be fine, but two hours later when it's packed with hot sweaty bodies may be a little different especially when dealing with tiny bandwidths. When the frequency shifts your pre-set bandwidth may not be dealing with it.
Try and get your source as close to the microphone as possible so you don't drag in unwanted signals.
If at all possible see if your location can be dampened somehow. At most venue locations this may not be possible, but if you're trying to record at home pillows, blankets and curtains draped around can be quite effective.