1) Thinking their music will sell itself.
I get it: Your music is brilliant. Your lyrics are unique. Your album art is original. However, it doesn't matter how great you and your songs are if people aren't listening. There are a ton of ways to go about doing this. Get your music on Pandora, iTunes and Spotify. Reach out to a few local blogs that review albums. Check out some of my other posts articles about being a professional musician. Whatever you decide to do, realize that post-production promotion requires just as much time and energy as writing and recording.
2) Not networking.
Get to know people in your local music scene. A good place to start is introducing yourself to the guys at the local music shop. You can also reach out to other area musicians via Reverbnation or Facebook. If you're confident in yourself and your music, you shouldn't be thinking of other artists and bands as competition. Instead, you should be trying to make as many acquaintances as possible. It opens the door for collaborations, opening act opportunities and more gigs. I can't accept every gig offer I get, but I have a handful of musician friends that I am happy to recommend instead. Networking will find you a lot more gigs than Craigslist.
3) Staying offline.
I have a friend who lives by the motto "If it's not on social media, it didn't happen". I think this is a little extreme, but I have embraced the effectiveness of social media (check out my previous article, 5 Career-Boosting Changes I Made in 2016). Social media is free advertising. It keeps your existing fans engaged and attracts new fans. If you don't want to have personal profiles broadcasting your life to the world, I get it. But having pages for your band or music is more of an expectation than an option in 2017.
4) Being impatient.
We live in an age of instant gratification. But despite what the music industry and talent TV shows want you to believe, people don't become stars overnight. Most of these people have been working on their craft their entire lives. Learning a good song takes time. Scales and arpeggios are boring. Writing a good song takes time. Building a polished show takes time. Building a fan base takes time. The music industry is a marathon; not a sprint.
5) Only focusing on performance.
Don't get me wrong: a tight show is important. Very important. But just like mistake #1 on this list, a great show is useless if no one is listening. I don't think musicians should be responsible for doing a venue's advertising and marketing, but there are a few simple steps you can take to increase your draw and build your fanbase: advertise your gig on social media, submit your event to the newspaper and even invite a local arts & entertainment journalist to review your show.
6) Being terrible administrators.
Like any good businessperson, musicians need to stay on top of administrative tasks: emails, contracts, newsletters, website maintenance, etc. Unfortunately, musicians are not known for being the most responsible folks. In my own experience, I've emailed plenty of musicians concerning collaborations and gig opportunities--- only to get no response. Not a good look.
PS. You should always have business cards on you too.
7) Being a slob.
For crying out loud, you are the entertainment. LOOK LIKE IT. It doesn't matter how great your band sounds---wearing cargo shorts and an untucked flannel shirt will detract from your performance. You can express yourself and your uniqueness while looking neat and professional. Maybe if you play like Stevie Wonder, this will be overlooked. Maybe. In the meantime, invest in a tie.
8) Playing too loud.
This is one of the biggest complaints artists and bands get on the gig. You can always turn your volume up, but coming out of the starting gate too loud will deter people from sticking around and subsequently annoy management. Spoiler alert: Unhappy management = not getting booked again. Know the room and who your audience is. If you were hired to play background music for a cocktail party, don't try to put on a rock concert.