I often attend the annual literary scifi convention. They have plenty of panels on how to increase sales. Becoming a popular "brand", e.g. Asian zombie novels,

I often attend the annual literary scifi convention. They have plenty of panels on how to increase sales. Becoming a popular "brand", e.g. Asian zombie novels, helps. Writing sequels and prequels to a modest central hit can help. Interestingly this group has lasted decades, from well before digitalization and the internet. The latter is a two edge sword, providing more market and competition at the same time.