I'm fairly bad at Romance writing, when it does happen to my characters. I've only had one good romance subplot, one where it just kind of...[i]happened[/i]. One moment the characters were doing their usual bantering, the next they were in a romance subplot. Compared to that, all the others seemed... arbitrary. Forced. 

My question, that I've taken too long to get to, is this: What's the best romance subplot you've had? And what advice can you give on writing one?