Having experiences (actually quite a few) in concert photography, I can give you some pointers, and hope it helps at least a bit.

I use Aperture priority (Av)
I don't use flash, unless it is waaay to dark for handheld shooting at my highest ISO speed. So if your highest acceptable ISO is 800, use it, and don't use flash. But keep an eye on shutter speed it gives you, of course. You know the drill: for handheld shooting shutter speed should be equivalent of your focal length at that moment. Crop factor must also be considered.
If shutter speeds are just barely over the minimum, I try to lean on to something or press the camera against something to stabilize it. I also wait for the performer to stop a bit, not move like a madman (very bad when shooting headbanging darkers).

In any case observe the lights and take the shot when the performer is best lit. Or when the light is the brightest, and just see what effect it gives you.

Always try underexposing. You have to know, that your camera might want occasionally to expose for the darker parts of the stage, while the lit part will be scorched... So experiment with exposure compensation.

OK, that's all very fine, but here comes the difficult part. Equipment.

When I give advice on concert photography, everything goes well until I realize I'm speaking for myself and for the equipment I use.

So I'm simply suggesting that you try to do your best with what you have. If you're gonna use flash, then try flash exposure compensation, too. You don't want to see deers in the headlights. Not pretty.

Oh, and another thing, before I mention the lenses. The lights have different color temperatures. I always set my white balance to the light on the performer's face. I don't want the faces to be yellow. So if the light on their faces is tungsten, then I go for tungsten, even if the lights in the back are halogen or whatever. In fact, white light in the background even turns a little blue and I just love it. But you never know. I've seen the usual lights (don't know the name) sometimes tungsten, sometimes halogen... Every lighting is different.

But I understand that it's easy for me to say these things, when my highest very acceptable ISO is 1600. I also use very bright lens. All my lens are 2.8L, 85mm fixed focal lenght and 50mm fixed I use frequently are 1.8. So I fully understand that if you don't have such lenses it's a lot harder not to use flash... But concert photography is a lot better without flash...

Most my concert photos are without flash... Never mind the Dog Eat Dog photo in my gallery. That one's made with flash, yes, because he was moving out of the light and I had enough of chasing him around, so I rolled my eyes and mounted the damn flash... the exposure of it (flash usually overexposes EVERYTHING) was pure luck...

Well, hope it helps. Try to do it as I told you, watch the lights and anticipate when a certain light will come on, then take the shot. And use every moment when the performer is still or slowly moving. Or exposing his face to the light... But always try exposure compensation, because his face might be a lot brighter than the surroundings, so the camera's metering might result in his face being overexposed.

That's about it.