Nora Roberts has an article entitled Crafting Romantic Suspense available online. While it doesn't have a rule list, it does have lots of info.
Deidre Savoy has a good how-to article, as well: her view being that romantic suspense is a full romance and a full murder mystery, combined.
From Becci Clayton, there are seven rules for writing romantic suspense (go here for the full article:
1. Romantic Relationship - First and foremost, these stories are romance.
2. Logical Suspense Plot - As with any suspense story, any error in logical suspense will not be forgiven by the reader.
3. Include Sexual Tension - ... a careful balance must be created between the suspense and the romantic relationship and the internal and external conflicts.
4. Insure the Correct Use of Language - ... A romantic suspense tends to be more spare and "clean" or "everyday" where straightforward romance tends to have a more emotional, intimate descriptive flavor.
5. Get the Details Right - ... If you are writing a story set in the early 1960's, DON'T put a laser sight on the murder rifle. They weren't available to the public yet....
Not as well defined as other genres, eh? Perhaps reading through books considered "Romantic Suspense" helps .... And before you think contemporary works, like that of Nora Roberts, Suzanne Brockmann, and Catherine Coulter, remember Rebecca by Daphne DuMaurier and Jane Eyre by Charlotte Bronte.
Oh, and there's a sub-sub-genre here: humorous romantic suspense, with Jayne Ann Krentz leading that pack.