Use a tripod, turn off autofocus and VR (prefocus before dark if possible but be prepared to adjust through the viewfinder or Live View once the show starts), shoot at ISO 100-200, aperture F8-F11 exposing for 2-8 seconds (longer is OK if it's a quiet show, but you'll probably run into noise reduction delays). If you don't have a remote shutter release, try using timer mode on a 2 second delay to avoid camera shake.