...lots (most?) of the actual work that needs to be do is "grinding" or "working against tons of hard/impossibly to remove friction": you'll never achieve any kind of flow doing this kind of work!

So the productivity multiplier advice is this: learn to cope and be productive at doing the kinds of work where flow is impossible/unfeasible!

(This way you'll not be already tired and behind when you get to the work for which "flow" is possible...)