I had a mattress topper, one of the ones that is held on with elastic straps. The elastic stretched out and didn't stay on very well. I took it and cut the straps off. I put a fitted sheet on my mattress upside down then put the topper on it. When I got it set in the right place, I pinned it in place, took it to my sewing machine and stitched them together. Then I sewed it from corner to corner in a big X. That keeps them together when washing.
I love it! My fitted sheet now has the padding and make the bed so much more comfortable. I have since gone to yard sales and picked up other toppers to put on my other sheets.
I use old mattress toppers when I'm making purses and tote bags. It doesn't matter if they're worn or stained, because they're hidden by the outer fabric, and it gives a nice quilted effect.
I finally used up all the crib toppers that I had with my son, and I need another one. (He's 33.)