G) Continue writing books in the same genre, and, if at all possible, have all of your books be in a series. (Note that the series need not be the type where the books have to be read in order, as with a trilogy. The important thing is that readers get to revisit the world of characters they've come to know and care about.) Books in a series reinforce each other. Books in a series give readers a subliminal sense that each book is more important than they would otherwise think if it were a standalone. Books in a series need only be sold to a reader once, and, if that reader loved the book, they will likely buy the rest in the series.
Multiple books are critical to success as a writer. Despite the few exceptions you can think of, nearly all successful authors have written multiple books. All other things being equal, (and assuming an author's books fit these conditions I've been writing about), the more books an author writes, the more successful he or she is.
Think through the basics of your series before you bring even your first book to market (Or before you change-up and re-market your first book).
Two more things to do: Make certain that your book covers communicate the "series" aspect. You want certain graphic aspects to be shared by all of your books, same size titles and font, same design theme, etc. The other is to have a "series identifier" in the title. The goal is that when readers of one of your books see another, they immediately recognize it as part of a series with which they're already familiar. To get a visceral sense of these series identifiers, spend some time looking at the "author pages" on Amazon of your favorite authors and see how the titles and graphics relate.
Stay tuned for the final installment of what to do about an under-performing novel.