Here's a great article by Mette Ivie Harrison: How To Write Romance (in Fantasy), published in OSC's Intergalactic Medicine Show. I think it's particularly appropriate to your question because it focuses on romances as subplots, romance combined with other elements.
Harrison starts out by rejecting "category romance" and obiquitous mishandlings of fictional romance. She has many valuable suggestions, but at the core, she suggests that a compelling romance should have:
- Characters who, while they may have deep flaws, are also deeply good (because romance relies on the characters being sympathetic to the reader).
- Characters who have a unique need for each other - but not dependence. Each one elevates the other in some (possibly unexpected) way; that drives home the romance. But if one character can't live or function without the other, the relationship becomes a pathetic one which doesn't gain a reader's respect.
In terms of plot, she suggests these elements:
- An external obstacle - the major thing keeping the two characters apart. She writes:
As a writer, you should set up the world at the beginning that would make it difficult for the two characters to fall in love and get married. Then the rest of the novel is playing this out, and you as a writer have to figure out the way out of the pit along with the characters and the reader.
- Having the main characters make mistakes; this is the stuff of drama and tragedy, which gives weight to your characters and story.
- Sacrifice, similarly, is the classic way to portray monumental choices and character development. It's a great capstone to a story, cementing life choices in a focused dramatic decision. Harrison writes:
I think this sacrifice needs to be on both sides for me to love the romance. If the sacrifice is unequal, I always end up wondering if in a couple of months, one of the couple will decide it wasn't worth it, after all.
The entire article is well worth reading. (She uses many examples from Pride and Prejudice, which is even more well worth reading.)