I've discovered these bed sheet suspenders this week, which you use on the underside of your fitted sheet to keep it taut. I currently have a thick topper on top of a deep mattress, and even with a deep sheet it was struggling to stay flat between washes.
These were £8 for four on Amazon, and much better than having to persevere with crumpled sheets or spend £30+ on an extra deep fitted sheet.
It's the kind of random thing that I would read about on Mumsnet, so thought I'd share in case of use to anyone!