One of the events I have been involved in over the last few years is shooting high school plays. Since these are high school, and I know most of the individuals in charge, there is a little more latitude than you may see on a more professional circuit. But I think the concepts are the same. Also, while I haven't really shot a concert I'm pretty sure the same rules can apply.
The assumption is that you do not have any control over the lighting. No speed lights or octa banks. So the light is probably not going to be good. At least not consistently good. One minute the whole stage is lit and the next minute you have a spot light beaming on the main character. That change in lighting intensity can really cause you issues. To handle these issues I think there are four key focus areas.
The first three of these are all of the components of a good exposure, the combination of ISO, shutter, and a fast lens. To get good exposure you're probably going to have to shoot wide open and at the slowest shutter you can. This only leaves the ISO as the remaining variable. If your camera doesn't shoot well at a high ISO you may have to consider a faster lens or trying to concentrate on a slower shutter and not shooting much action. Another option may be to only shoot when the stage is well lit. These options are pretty limiting but may be the only choices you have.
Another important and often overlooked aspect of shooting these images is positioning yourself in a good location to get the quality images you're looking for. As I said, if you're at a public concert and don't have permission to move around you may be limited to your assigned seat and you will just have to make the best of it. But if you do have permission to move around or have some influence on where you're seated, be sure to pre-plan and pick the best spot you can.
There are also some some additional considerations that you need to think about. Do you shoot wide or close? Using a zoom may take care of this, but even then your zoom may be too close or too wide. A real wide zoom will distort, especially around the edges. A real close shot will compress the background and give you a narrower depth of field, especially if you are shooting wide open. If you're shooting a play and it's important to get everyone on stage then you may want to shoot extra wide, even with the distortion. One answer to this is two camera bodies each having the appropriate lens.
Another thing to remember is not to shoot into the light. This sounds like a no brainer, but there could be some backlights on the stage that you may not be aware of until they are turned on. It's especially important if the subject is suddenly between you and a spot light. It may be a cool shot to get the silhouette with a backlight though. Just be aware of possible issues with flare and the possibility no detail in the subject.
Along the line of shooting into the light is how quickly you may go from a well lit stage to a spotlight on the subject. My experience has shown that you can easily blow out the subject in this situation. Just be aware and close down your aperture or speed up your shutter at least a stop. Otherwise, there will be no detail to recover in post and you may have a totally white face. Then remember to change it back when the lights come back on.
When the lighting conditions are changing so quickly and/or there's lots of action or movement I would recommend shooting lots of images. It doesn't cost you anything and it gives you many more opportunities to get something you like. You may get a blown out image and then the next one is great.
Finally, this is one of the few times I would shoot in the Automatic White balance mode. This way you can concentrate on getting the image and have one less thing to worry about with the ever changing lighting conditions. You can always adjust this later, especially if you're shooting raw.
Hope this helps. Happy shooting.