It's very rare to find a venue that doesn't have a desk that can send phantom power, but I always bring a cheap behringer phantom power box with me just in case. This plugs into the mains, has an XLR connection in and out and can send 48V or 12V to a microphone. And yes, choreography (of the loosest sort) and excessive stage volume are very much the reason for me using a clip-on! In an ideal world, I'd have a Beyerdynamic M88 on stage (recommended by a BBC tech for saxes), but it would be totally impractical in a rock and roll setting.