Rolling up t-shirts or a jacket under my wrists while I type has been tremendous for avoiding wrist pain in the day.
For my back, I practice rolling my hips forward (roll your groin towards the chair), which changes which muscles are use and can alleviate low back pain.
After a lot of screwing around with desks and trays and other things, I finally just gave up and started typing with my keyboard in my lap. This has worked quite well now for a long time; everything seems to be in the correct ergonomic position, with a basic ergonomic split keyboard (though even the non-split keyboard on my laptop isn't too bad). For mouse usage, I just use it infrequently enough for this to be practical, by having a keyboard-heavy setup.
No, I lean back in my chair. There's also some evidence to suggest that the "chaise longe" position is quite ergonomic, though I'm not quite that far back. I'm fairly tall, so sitting pretty & straight as the pictures show is generally not an option as even with chairs adjusted to full height I can't sit with my knees at a right angle.
For my back, I practice rolling my hips forward (roll your groin towards the chair), which changes which muscles are use and can alleviate low back pain.