A previous poster was suggestion S shutter priority.. Probably at least 1/300 if you're zooming.
You could also try A aperature mode, set to your camera's brightest aperature f/2.8. The risk of A mode is it may give you a slower shutter speed, resulting in motion blur.
You also want to shoot in RAW instead of just JPG , even with your fast lens, you will probably wing up wih ISO 1600 or 3200 and you can get better high ISO pictures from RAW.
I would also suggest setting the camera for spot or center focus and metering, those usually help keep the camera more responsive.
If possible, play around. Its not bad to try A S and Auto during the show, sometimes you can "outsmart" the cameras Auto, sometimes you can.