We've all been there. It hurts your heart when you have to crank your ISO. You just want to go back and slap the lighting guy, push him aside, and turn up the spotlight (if the venue has one).
The bottom line is, you're just not going to get spectacular shots out of a gig with very poor lighting. This happens mainly in dive bars and smaller venues (<500 capacity), although the lighting at club-type venues can also be a bit dim.
So here are a few pointers on how to deal with less than ideal lighting at a show when shooting at f2.8. If you have anything to add please feel free to comment!
1. Look for the brightest area on stage. When a performer steps into the light snap as many photos as you can. Be patient, especially if you are able to shoot the entire show.
2. Crank your ISO so that you can have a fast enough shutter speed to eliminate or decrease motion blur. I'd rather have noisy, in-focus photos than shots fllled with motion blur.
3. Take wider shots. In darker situations when there are several shadows, wider (further away) images put the scene together a little more nicely and are easier to work with when editing.
4. When editing the images, increase the exposure a bit along with the highlights. The white balance will, most likely, be off; so play around with the colors and tone curves a bit. It can be fun, and gives you an opportunity to be creative.
5. When a color is too saturated (saturated red light is a concert photographer's worst nightmare), your best bet is to convert the photo to black & white.
6. Snap on your f1.8 lens, if you have one. I mainly use an f2.8 because I like being able to zoom, but I have an f1.8 for worst case scenarios.
7. I don't recommend this, but when nothing else seems to be working in your favor, use flash. The reason I don't recommend it is because flash can distract performers, annoy fans, and take away from the feeling and atmosphere of your photos (using the available light makes for a better ambiance and provides more of an "I feel like I'm there" feeling to their viewers). But in desperate situations it can be necessary. ***BUT keep in mind that "no flash" is often a restriction when shooting a show.***
8. If you're sitting there editing your photos and you find yourself heaving sighs of disappointment, and you know your way around Photoshop, add a cool grunge or antique-type overlay. It'll add more character to a photo and take away from the fact that the lighting might not be awesome or the photo isn't completely in focus.
9. A local venue here recently got a strobe light installed in the ceiling. While you can't time your shots with strobes, take as many shots as you can when the strobe is flashing (the "spray and pray" method) and usually you'll get at least one shot during which the strobe flashed.
If you have more tips please feel free to add them in the comments below!