Most stage microphones you will encounter in live performance, such as the Shure SM58, will be what’s known as a “dynamic” type microphone. Dynamic microphones are relatively inexpensive, require no electrical power to operate and are usually built from a few robust parts. This makes dynamic microphones ideal for both budget recording and live performance.
All microphones have a pickup (or polar) pattern that dictates the amount of sound they will “pick up” from various directions around the microphone (see what they did there?). For instance, an omni-directional pickup pattern means the microphone will pick up all sounds from any direction, which is great for natural-sounding outdoor-location film recording, but not so great for miking onstage with guitar amps and drums producing high-level volume.
Pickup patterns for modern stage microphones are therefore usually more selective, only picking up sound from a certain direction. This pickup pattern for a dynamic microphone, such as the SM58, is known as a cardioid type and will only pick up sound from the front and sides of the microphone and “reject” sounds from behind—the place where you usually hold it. 
The cardioid pattern is ideal for live music applications, especially vocal microphones, as the sound coming from the monitor wedges on the ground will be will not be picked by the microphone too much, due to the microphone’s pick up pattern. Having examined microphone theory, you should now be able to apply what you know about microphone types and pickup patterns to help with better microphone technique––therefore avoiding feedback.
The first rule of microphone technique is that you should never “cup” your microphone if you want to avoid onstage feedback. “Cupping” a microphone involves holding the microphone too close to the grille (the part you sing into) or, even worse, covering the whole grille with your hand. By enclosing the grille (and therefore the capsule) of the microphone in your hand, you will alter the pickup pattern of the microphone. Sound will be funneled into the microphone’s diaphragm (the part that converts sound into electrical signals that can be amplified) through the small gap in your hand. The diaphragm, which works by sensing the differences in pressure between the front and back of the microphones capsule, will be confused, and the directional characteristics of the microphone will change. Yes, you will achieve a higher volume for your vocal from the microphone by cupping it, but you also will increase the risk of feedback.
In fact, feedback is more than likely to occur because the cupping will be boosting the very frequencies that are more likely to be produced in a feedback loop. For instance, cupping a Shure SM58 will usually boost frequencies around the 5KHz range, which is a nasty, nasty frequency—even more so at high volume levels. (If you wish to find out how unpleasant this frequency sounds, go to YouTube and search, “5KHz / 5000 Hz Test tone,” turn your headphones or speakers up and press play. The results will not be pleasant, so be warned.) Avoid this type of feedback by holding your microphone halfway down the barrel, making sure that no part of your hand touches the grille.
It’s the angle: Good microphone technique involves not only handling (not cupping the microphone), but also knowing the correct way to sing into the microphone. Your microphone should be at a 90° angle to your mouth in order for the microphone to pick up as much of your vocal as possible. Try to avoid singing “over the top” of your microphone, i.e. holding it upright near your mouth like a lollipop, as you will be singing into the side of the microphone and there will be less of your vocal being picked up.
