1) Keep an open space between the thumb and the side of the hand. (This will force a good hand position anyhow, else I can’t get my fingers where I need them.)
2) Keep the fingers relaxed. Not limp — ready to be used, but supple and relaxed.
These two things taken together seem to give me almost everything. (Simple != easy.) A lot of the rest of it is just keeping my attention on what needs to be caretaken, which is very hard since I haven’t “chunked” the tasks yet.
This masterclass with a woman named Catherine Michel who appears to be a big-time mover and shaker in the harp world, was also a big help to me, just at this one moment, where she specifies keeping the fingers low to ease the pivoting of the hand around the thumb. (The theme-and-variations piece that this girl is playing is also one of the nicest things I’ve heard in a long time.)