1) You must have a strategy that allows you to nurture readers and fans so that they become customers when the time comes
2) Diversify your income stream. Figure out the best 'Author Business Model' for you and your financial goals and run with it.
3) Don't forget the additional products OR services that your audience would love to get from you. This could be short stories about your fictional characters, or planners/journals/notebooks for non-fiction work.
4) Focus on creating an experience for your readers so that they look forward to your next release.
5) What platform do you want to publish your book on? Get familiar with the pros and cons of each (print, eBooks, audiobooks ) and find out what will work for you. Do what works for you and what makes sense for your overall goals.
6) Be patient! It takes some time to build a readership and book sales at a consistent level. Make sure you stick with it and start this process even BEFORE your book is done and ready to publish.
7) List ALL of the skills you used to write and publish your book, figure out which ones you enjoyed the most, then start freelancing.
8) Consider Author newsletter swaps, Book collaborations, and speaking at book conferences, as a way to get in the room (and sell books) to wider audiences. This works best if you have taken the time to really get to know what your audience likes.