For whatever reason, I just got a whole bunch of blog posts by Steve Barnes (who just generally rocks) dumped into my reader, so I'm going through a lot of good stuff. I ran into this one, which struck me as particularly useful for anyone writing romances, or just writing about relationships between people.
Whether you're looking to write a believably solid relationship, or looking for reasons why a relationship might crash and burn, Steve has a pretty comprehensive list here. Great bunny chow. :)