I buy the sheets with deeper pockets for my thick mattress. Seen them anywhere from 13' to 17' but may also have them larger.

Sheet suspenders work well.

I bought sheets at target for my kids' beds (which have thick memory foam toppers on them) and they are really nice and deep. They have a double elastic on every corner which seems to make it easier to put on plus keeps it on. They aren't cheap though (IMO-maybe I'm a cheapskate) so right now each kid has one set and for washing, we take them off early and get them all washed and dried so they can put them back on at bedtime.

the bonus-no folding of sheets.

I'll have to try these!

Fitted mattress pads and fitted sheet with large gussets work for me. I have two fitted mattress pads on each bed. Sure keeps them clean and looking like new.

Oops I wrote 13' to 17'. I meant inches! That would have been one big sheet.

I didn't even catch that. Sized for the Jolly Green Giant!

I had that problem and as weird as it sounds, I put the sheet first on the corner that kept slipping and made sure it was snuggly settled in, then next, the diagonal corner, then the one on the same side as the diagonal, then the last one. It seemed to stay a little longer between adjustments. Just try to snuggle them in the corners as well as you can.

I folded a square of fabric diagonally and sewed it to each side of the corner when I had some difficult sheets. Kind of like a bigger than average gusset. Are the sheets a woven cotton? It sounds like they are stretchy.

The sheets are 100% woven cotton so no stretching. How big a square did you start with?