There are many ways to kill a music career- setting the star singer's hair on fire, discharging handguns onstage, etc. Here are three of the more common methods I've seen to commit professional hari-kari:
1. Not being you
Their are a million almost-as-good-as-the-real-thing Britney Spears, Kurt Cobains, and Paul McCartneys. The bad news is, Britney Spears, Kurt Cobain and Paul McCartney are the best at what they do. Because they're who they are, and no one else is. You are you, and you're the best at it in the whole world. Don't ever forget to leverage that every chance you get.
2. Lack of training
While being original is what we all strive for, that doesn't mean you shouldn't learn from others. I spent huge chunks of my career learning how to do things wrong- I get sick thinking about how much time I lost failing, and wonder where I could be now if I'd found the right people to help me avoid common mistakes.
You really can teach yourself most things, but it'll take far more time than if you just learn from a professional. Get a day job, pay the money, and speed up your career. The faster you learn, the bigger your edge.
3. Being an idiot
It seems simple. Don't yell at the club owner. Don't make your set run a half hour into the other band's time. Don't smoke pot at that children's party you were hired to play (true story)!
Most of us sensitive musician types have active imaginations- let's use them to imagine what others might want around us, and treat 'em like we'd like to be treated. Even if that means taking a few lumps for the sake of societal harmony. You might be surprised how far people will go out of the way to help someone they like.