I'd actually guess that one of the biggest, most common mistakes new writers make is getting the ratio of new writing to promoting correct. When I was starting out as a novelist, there were a lot of writers who went all in on promotion, and lost their momentum because their readers had nothing new to read. Others were head down in solitude writing the next book, but then their publishers cancelled them because their numbers weren't there. It's a tough tightrope. Doing things as a group, where each author can promote the other authors in their area to their local events is probably the best way I found to walk that tightrope. That, and participate in the publisher-sponsored book tours. It might not have sold many of my books, but it sold a lot of books for the publisher, and they remembered that I contributed, which helped with future book deals.