I've just begun trying to list techniques used to continue the sexual/relationship tension in novels.
1. Characters disagree on something fundamental/are fundamentally different. Their arguments can metamorphose into sexual tension at some points and diffuse it into true anger at others. Or two characters with difference in social position or culture can believe they understand each other then come up against misunderstanding, which they must resolve. Seems like a flexible way to go about things; one could get a lot of plot twists out of this sort of relationship.
2. The Big Misunderstanding, a term I first saw on the All About Romance website. Mostly used more as Deus ex machina to disrupt a Perfect Love; doesn't often grow from inside the characters. Possibly a subset of 1.
3. Parceling it out. Making the characters wait before they can have sexual contact, or giving them little bits over the course of the book, either through circumstance or because they have reservations of one kind or another. I'm using this one, at the moment.
4. Outside events. Ending the sexual tension, but having other tension take over, such as head-hunting mutants pursuing the hero and heroine across the prairie.