Successful romantic suspense author Lisa Gardner's article “The Seven Secrets of Romantic Success” suggests starting in the library. Research your subject matter. You must understand the kind of background that goes into criminal suspense novels and mystery thrillers. You need to demonstrate that you understand the way firearms are fired and that you know how medical procedures and postmortem examinations are handled. You should have a solid knowledge of how the organizations and institutions that appear in your fiction operate in order to write about them convincingly. Some of this research can be academic, some of it you may want to experience first hand. You also need to understand what makes a compelling villain. You may want to research the criminal mind and how people are murdered.
Create compelling characters. In order for your readers to care about (and buy) your writing, you need to craft heroes and villains that are more than just interesting: they need to compel your readers to pick the book up again and again until they finish it.
As for the bad guys, writes successful author Deidre Savoy in her article “Writing Romantic Suspense," “Gone are the days where a writer could describe her villain as just plain crazy.” Your antagonist has to have compelling and understandable motives, needs a connection to one or the other of your romantic leads, and has to represent a significant challenge for them to overcome.
Your protagonists, on the other hand, need more than just some sparkling chemistry. Both romantic leads need a stake in the events of the plot, and should learn something about themselves through the course of the story and their interactions with one another.
Craft a tense and suspenseful tone. Focus on more than just the scary abandoned buildings or creepy old mansions. Characters reflect tone by how they react to events are they unfold: if the characters are convincingly tense and uncertain of what's around the next corner, chances are your readers will be as well. Throw some twists and turns into your story, surprises that your readers will not see coming.
Connect the growing sexual tension between your romantic leads with the evolving suspense storyline. As your characters discover new hints and clues that unravel the mystery behind the story, your characters should be feeling the tension between them rise accordingly. As things get more dangerous for your heroes, it should get steamier for them as well. Gardner writes that this practice “ratchets up the tension, keeping the reader flipping pages until the late hours of the night.”
Give your readers a conclusive and satisfying ending that will leave them feeling good about what they've just read. Provide a satisfying ending for the love story: "happily ever after" is considered the standard. And put the bad guys away; whether you put them in the ground or behind bars, you need to leave your readers with the impression that the heroes defeated them.