The one thing that has made the biggest impact is making the decision to stop treating writing as a hobby and approach it in every way with the mindset to make it a sustainable career. That entails sticking to a regular writing & publishing schedule, and constantly re-evaluating everything else in regards to ROI. Anything besides writing (promotion, social media, collaboration) needs to somehow advance my goals; the plan is always changing, since what works in this industry is never a stagnant thing, but my constant is that writing/publishing needs to be the priority and bulk of my efforts.
Of course, things that were important when I started as an author are not so important now. I think the ability to recognize when you need to re-assess priorities is an important ingredient of success. At one point, growing my social media presence was a huge deal to me, and I soaked up everything I could on that subject. Now, not so much. Focusing on SM was fun, but when sales faltered, I had to take a hard look at what I was doing; was spending hours on SM selling me books, or was it just inflating my ego? Isn't the ultimate goal to get people to read my books? So yeah, the decision to dedicate less resources to SM was a good one as far as the overall goal (write books/publish books). Same goes with learning about how to micro-manage my mailing list, or becoming an expert in Facebook Ads. Sure, spending hours twerking my mailing list, trying all the techniques, becoming super-awesome at running FB ads can be a great thing, but when I look back on a week's worth of work time and see that my actual writing productivity was only 10% of my time, that's when I know I need to come up with a new direction. Even another example, sometimes I'll get tied up making covers, looking at images, working on new concepts for graphics & stuff...not so much anymore. In the past, I made my covers because I had to. Now, as things have evolved, that is just another task that I can likely farm out in order to focus 100% on what I have found is my best chance at success-- regular writing/publishing.
For me, there are a LOT of things in this industry that can take up your time as an author. Plenty of them can help you sell books. Plenty of them are just a time suck. The constant that remains (for me) is write good books/publish good books, as frequently as possible. Decide what YOUR ultimate goal is, and be honest with yourself and ask if something you're spending a lot of time on is actually propelling you towards it.