Just wanted to add to and support what Eric said about noting when a mic dies. If you keep track of this, you'll eventually see the pattern, after a while you can eliminate a lot of deads.
This summer I did an open air theater. For the first four shows, a girl which went on always had a crackling mic. Four shows.. wow. It was a mic swap between her and a guy which was on stage before her. It always worked on the guy. But this guy (a teenager) was so stressed out that he'd miss his unmiked entrance way later in the act, that he just ripped his headworn and pulled the cable every time to get out his beltpack. After the fourth show I asked the (volunteer, no sound experience except this play once a year) "A2" what's going on? Is he just ripping of his headworn? And the "A2" said Yup! After a talk to the actor and the A2, there were no more problems.
You need to speak to the people using the mic, and the people who works directly with the person who uses the mic. They might see something the actor doesn't recognize themselves. Always do a "crackle-check" with headphones or a speaker before you hand out the headworns for the day. They don't break when the actor is putting them on, but rather when they take them off, and if it's kids, they tend to play around, fall, get some hands tangled into his or her friends cable etc, generally break something the second they have the time to do it.
This was more about physical abuse of the equipment though. There's factors like sweat and such which you can't control as easily unless you use some countermeasures.