1) Book Excerpts.
At the end of every book, have samples and teasers of your other publications. If they made it to the end of your book, chances are they enjoyed it and would gladly buy more from you. Hence why you should advertise other releases.
Banners should be everywhere. Your blog. Your website. Your social media accounts. Banners should advertise your publications for every reader to see.
3) Newsletter Promotion.
You send a newsletter every month to your readers. Why not have ads strategically placed for readers to see when flipping through your monthly newsletter content? Kind of like Facebook ads appearing in news feeds.
Now, I know most authors don't see the point in giving something away for free, but they couldn't be more wrong. A free book will get downloads, reaching out to more readers, who will see your ads for other books at the end. They may also sign up to your newsletter, expanding your reach even further. And if the free book is the first in a series, they'll be compelled to buy the others in that series, which would generate actual money.
Offer to collaborate with other authors. You'll tap into their readership, and they'll tap into yours. You are essentially advertising yourself to their readers. Collaborate on a short story, and give it away for free. You both promote it. Straight away, there is double the outreach. Then of course, you are tapping into their readers. It's a win-win.
There are of course other ways to advertise as an author. Remember, it's a business. Whenever they buy a book, subscribe to your newsletter, head over to your site or social media page, never miss out on an opportunity to advertise your other releases.