The advice would be the same for any kind of fiction. Keep writing, and keep sending things out, not to friends and relatives, but to people who have the power to buy. A lot of additional, useful tips could be added, but this is fundamental.

The Dracula Tape (Warner June 1975) / (Ace Jan. 1984
"Deathwomb" Poul Anderson (nv) Analog Nov. 1981) (short fiction collection; only 2 original/uncollected stories); (available online) as a Baen Free Sample from Berserker Death omnibus

Berserker Base (Tor March 1985); Mosaic Berserker novel with several guest authors contributing original stories; Saberhagen wrote the overarching story in segments between them using the Niven story as the novel's fulcrum point:
"What Makes Us Human" Stephen R.

Saberhagen also wrote a series of vampire novels in which the vampires (including the famous Dracula) are the protagonists and a series of post-apocalyptic mytho-magical novels beginning with his popular Empire of the East and continuing through a long series of Swords and Lost Swords novels. F.