Instead of answering, I went through all the responses thus far and made a spreadsheet of the answers. (This is absolutely non-scientific and possibly inaccurate, plus there's no data as to how well these strategies worked, but hey, I was curious and thought maybe others might be interested too.)
I also have a list of "Other" answers that didn't fit neatly in these boxes or struck me in some way-- I'll link those next.
@johnyNocash mentioned pre-sales with extras (I also do direct sales with goodies), but what really struck me was his music-oriented strategy. Very cool, and I hadn't heard of that before.
@Firlefanz mentioned BookFunnel promos which I also do, and using Fussy Librarian, which I've heard about but haven't yet tried.
@catchingshadows caught my attention mentioning Discord servers, as I'm on Discord a lot too.
@anderlandbooks mentioned making pictures and videos which sounded very cool to me.