I'm wondering if it's based on how much damage you do, or simply how many times you cast the spell. For example, should I dual cast Incinerate (for triple damage and probably a one-shot hit), or should I just cast it two or three times separately?

Based on damage dealt, IIRC.

And nowhere near fast enough. Before I modded it for myself to advance at a rate more comparable to other kill skills, it advanced at an agonizingly slow rate. I think I can count the number of times I've used a trainer for One-handed in all my play-thrus on one hand (pun not intended )... I've lost track of how many times I've had to train with Faralda or Sybille for Destruction to catch up.

And that's on top of Destruction's "requires enchanting abuse to be effective in the first place" problems.

I've been using the Balanced Magic mod, which makes Destruction awesome, so damage is not a problem. But it doesn't change the rate it increases. I'm already at 90 so I can't train with Faralda anymore. I'm trying to figure out the most efficient way to get to 100 now.

Are you willing to delve into the CK yourself? There is a way to boost the skill increase rate...

Nah, I'm close enough already, just curious what the rules are for how it increases.