1. Be sure you have a foolproof crucible plot. In a romance, your hero and heroine must be in a situation they can't escape. If they could walk away and don't after the first big conflict, then your readers will think they're stupid and deserve each other! Put them in a situation where walking away would cost them far too much.
2. Your book should begin with "why is this day unlike any other?" Get to that quickly. If it is just another day, then it's dull. Something monumental must happen that day -- within a few pages of the book -- to hook the reader.
3. Peel your main characters like an onion. One layer at a time. Don't tell us their whole backstory in big clumps of pages. Feed it to us in small doses, telling us only what is necessary for us to know about their history in order to understand how they are handling the present situation. Readers need to get to know your characters a bit before they will be interested in what happened to them in the past.
4. Secondary characters should support and play off of the main characters. They are there to reveal more about the hero and heroine or to throw up roadblocks. Don't allow them to have their own story that has little to do with what's going on with the hero and heroine. Keep the focus on your protagonists.
5. Finally, and most importantly, "show, don't tell." Show the story through your main characters' eyes, ears, nose, taste, and touch. Don't tell us things through your author viewpoint. You aren't part of this story. You aren't a character in this book. Don't tell us things about the characters -- show us the characters by staying in their viewpoints so that we experience the world through them -- not through you.