I remember taping a lot of Valentine's Day special movies for my mother on the Hallmark Channel and Lifetime (she was gone all day and kind of assigned me to this duty. I considered it a sort of Hell). And I hated them, because they were so damn predictable. There was one---I it was called The Wedding Planner---that really took the cake for me.

The lead woman fell in love with this guy, etc., etc., and events lead to drive them apart, to the point where they were each going to marry different people. The girl's parents arranged a fiance for her (they were very traditional) and the fiance was a very good guy. They almost got married, and it was hinted that she could be happy with the arranged fiance. I was actually surprised that this movie actually pulled off a good twist---

---Only to untwist it all again in the next five minutes, with the woman going to the first guy's wedding, and they go off together. And the ditched spouses-to-be were oh-so-okay with it... I remember being pissed that Hollywood gave in to the desire to give it the typical happy ending...

I disliked Click for the same reason. I still think that movie should have ended with the main character realizing that he wasted the best parts of his life and not have the chance to fix it. It would have been a lot more meaningful than the fact that he got to fix everything.

But getting back on topic, I think that this rule should apply not just to love triangles, but to other aspects of writing. If a character has to make a choice, make it a tough one. A classic example is the Sadistic Choice: a villain has two hostages and will let one go, but kill the other. Done right, it will tear the audience as much as the hero with which one he chooses.

A straight example would be the climax in the first Spider-Man movie, where the Green Goblin was forcing Spidey to choose between Mary Jane and a trolly car full of kids. Props to this example with Goblin making the choice harder by dropping them both so Spider-Man had only seconds to think and react.

Along with pace and tension, I think characterization needs to play into it too, but that is obvious advice.