>>8866076Experience and training. My karaoke before this one was closer to 6.5 hours, and I've had days where I've needed to sing for about 12 hours before. Everything related to your voice has to do with a set of muscles, and like any muscles, it's long use and proper conditioning that keep them working. And like the anon above pointed out, I've got lots of experience. Like any muscles, with proper use, they get stronger, and you can sing more powerfully for longer.
The biggest thing is maintaining proper vocal production. You don't want to introduce strain anywhere if you can help it. So even when I'm singing something like Tenacious D or Gethsemane from Jesus Christ Superstar, I'm still fairly relaxed in the muscles around my vocal apparatus. So for example, I don't clench my jaw up, strain my neck, etc. It's also important to warm up before beginning, so that you're loose and relaxed. It's just like stretching before and after any other exercise. If you're doing things that hurt your voice, it's best to back off.
I realize that's not super helpful, but basically, the tl;dr version is don't push, and your voice gets stronger over time. Do breathing exercises if your voice is starting to feel tired.
