Sometimes I have days of low morale where I don't get much done, and don't try to force myself to do things because I know my morale is low and I'll likely fail. I'm experimenting with a few different strategies for cutting down on low-morale days... I'd like to have ... better motivation (which might allow me to work on things with less willpower/energy expenditure),

Morale, and reducing the need for willpower / conscious effort, are things I've had success with using self-image changes, e.g. inspired by Naruto :) So...

those things seem to me to be more about trying out a wide variety of techniques and empirically determining what works.

... I'd say paying close attention to how you see yourself and your place in the world during times of low morale is definitely worth experimenting with. I'd actually be quite surprised if there aren't variables at play there which, if changed, would cause changes in your morale.

I avoid listening to Britney Spears, for instance, because I don't want to be the sort of person who likes Britney Spears.

Hah! It's funny you should mention that! Liking Britney Spears was one of the first intentional changes I made to myself at the time I mentioned in the post when I was 16 and trying to be self-image-free. It worked; I realized I naturally quite liked most of her hits, and I always perk up when her music comes on the radio. It's nice not to have to hate it :)