As more and more of the old rules fall by the wayside and more things are decided case-by-case, it's now becoming common for self-published books to become traditionally published. It's a trend with literary agents as well, as they may come on board once they've read an excellent self-published book. Publishing industry trade media report that about half of publishers today acquire titles that were self-published originally. It says a lot about how the industry is changing and the ever-increasing quality of self-published books. Authors today can take several routes to publish successful books.
It's also worthy to note that not every self-published author aspires to go traditional. Some do very well on their own and prize independence and a full share of the profits. But for authors who would like to work through a traditional publisher, the percentages are becoming large enough that it's a realistic goal—the most effective way to improve your chances is to write an excellent book. You'll also need to pay to have it professionally edited and make sure you hire a cover designer to make your work look complete. Cutting corners on any of these can count your out quickly with traditional publishers.
Nothing sparks a publisher's interest more than book sales. Therefore you need a book marketing campaign that helps you attract readers and fans. You can do a lot of it online and through social media, but a traditional publicity campaign is almost always more influential. When you can show media coverage like articles and interviews, your stock will rise in the eyes of conventional publishing houses. There was a time when publishers expected to grow and promote their authors, and many still do, but having someone arrive with those priorities already accomplished is of significant interest.
Don't give up quickly if you want to move from self to traditional publishing. It takes time and perseverance for many authors. Only a sustained effort will accurately show whether you and your book can be picked up and published traditionally. If your initial efforts to attract an agent and publisher fail, don't give up immediately. Your second self-published book may be more successful than your first one and provide the basis for another try. Publishing is one of the world's most competitive businesses, and those who prevail are resolved and have the drive to succeed. It often takes tenacity to be noticed.