I wish I'd been shown the 'monk's grip' for holding a pen much earlier in my school career, as that would have helped my pain/arm ache enormously. I pushed through and made myself write quickly and neatly, but at such a cost.
I think there should be a push for legibility, not perfection. Many children learn in primary how to write very neatly, when required, but not at speed. A fluent, joined, speeded script that is legible is much more useful that a lovely, very neat, slow script that is just useful for 'best'.
Lots of space needed at desks, and appropriate height. Room to allow the arm to move from the elbow and shoulder, not just the wrist.
Time to do it properly! So much copying from the board, with very little time to do it, and children end up going faster than they're capable of. There needs to be a better transition from primary school, to build up speed gradually.
Enough paper/space. Some schools scrimp on photocopying so that worksheets, exam practice papers, homework etc are all done half size, so writing has to be squashed into small space, which doesn't foster good habits.
Time to practice - maybe a daily session in form time. Some of the activities used in remedial handwriting programmes could be incorporated, like worked large, making written patterns in rhythm to music to promote good 'flow' and arm movement, making sure children remember the correct letter formation and joins.