My ideas come from pretty much what I've been recently listening to, and kinda mash it together with something I've watched recently, either matching up the feeling or the meaning I get from both. I tubedub AMVs with different songs to see if it still fits the themes or different videos to the same song. Then my mind switches the songs and the videos around until I feel 1: they're "relevant"(or not) enough to each other, 2: there's enough "footage"(show/character) to do something with, and 3: I can storyboard 90% of the video in my head at least.
Such as, I've been listening to a bunch of tool lately and reading a bunch of homestuck, and want to do another AMV to tool:
Madoka - Stinkfist (turns to)
Madoka - Jambi (turns to)
homestuck - Jambi (turns to)
homestuck - Lateralus (turns to)
Hellsing - Lateralus (turns to)
Hellsing - Parabola (turns to) *hell, Tool and Hellsing go together like DBZ and Lincoln Park
Full Metal - Parabola "...this body holding me/ be my reminder here that I am not alone..."
Its almost like they were both based off each other! I'd make it except I kinda stopped watching around episode 23, and I don't think I could edit from a 50+ series
tl;dr I use Cuil theory to find what songs to use
"Uhmmm... You know... it was at that point that I realized that maybe Thierry wasn't actually a film maker, and he was maybe just someone with mental problems who happened to have a camera. " -Banksy