Study the romantic suspense genre thoroughly. Get a feel for the structure, pacing, development of characters and degree of sexuality.
Consult websites like Suspense Romance Writers (see Resources), which feature different authors and provide reviews of new romantic suspense fiction.
Decide who your leading lady and leading man will be. Create character profiles--how they look, how old they are, what kind of jobs they hold, whether they have family or siblings, etc. Your heroine should be smart, accomplished, well educated and able to take care of herself. Leading men should be intelligent, in good shape, great conversationalists and have a sensitive side. Neither of them should be in a relationship, nor should they engage in questionable or dishonorable behaviors.
Identify the villain. This individual needs to have a strong reason to want the heroine out of the way and should be relentless in trying to frighten, discredit or kill her. Don't make the villain cartoonish or stereotyped. Whether or not you expose the villain early in your novel or draw out the intrigue to the final chapters, the villain and his deeds need to be a constant presence in the storyline.
Come up with a major conflict that threatens to keep your couple from getting together. Example: The hero suspects the heroine's brother is behind the attempts on her life because he wants to take over control of the family factory. He presents his evidence to her, but she angrily refuses to believe him and accuses him of being jealous of her family's wealth since he grew up a poor orphan. Create scenarios that could make it look as if the hero betrayed her trust, which could propel her straight into the "helpful" arms of the real enemy.
Decide where and when your plot will transpire. Experiment with different settings and eras until you find the one that best fits for your story.
Hit the ground running with something exciting or dangerous happening in the first chapter. Romantic suspense plots typically transpire over the period of a few weeks. The heroine should meet the hero no later than the start of Chapter 2.
Create sexual tension from the meeting of the hero and heroine, and steadily increase it as you move through the plot. If you place them on opposite sides of a problem in the story, make sure it's not insurmountable.
Solicit friends to read your chapters in progress and give you feedback. Ask them to speculate on what they think will happen next. If they guess too accurately, it's likely that others might figure it out, too. Pull a surprise and do something else instead.
Review the submission guidelines of publishers you'd like to submit your work to before you get too far along in your writing. These can be found in annual publications such as "Writers Market" (published by Writers Digest Books and available through Amazon.com). Such listings advise you of publishers' desired word counts, which editor to query, and whether you'll need an agent to pitch your work.