On fixed hours, which I think is the underrated one here.
I moved from "send work when it comes up" to a fixed block and the quality changed within a fortnight, before anything else about the arrangement had changed. My read is that irregular work forces someone to context-switch into your business cold every time, and half of any short session was spent remembering how you do things.
A predictable block means they arrive already in it.
The bit I would add: I also stopped sending things outside the block. That was harder than setting the block, because the whole appeal of having help is offloading the thing that is annoying you right now. Queuing it costs me a little friction and buys the arrangement its shape.