Open-ended question(s) of the day: How do you come up with more powerful magic and magic items?

Personally, I have found myself to be very good at coming up with imagery, settings, environments, character personalities, etc, but not as good at conjuring up more powerful magics and such. Presumably you have to start simple and scale up from there, at least for any given storyline. For that particular story, only the most powerful magics are needed for the climax, but if you're planning a story arch that goes beyond a single story, you're probably going to have to come up with a whole lot more, which can be daunting.