Keeping a Foam Bed Topper Flat

You could sew (hand or machine) elastic strips across the corners and pull the strips under the mattress to keep it in place.

Or you could make something like a duvet cover using flat bed sheets to enclose the topper, attaching the elastic strips to the cover so that you could pull the strips under the mattress.

There are commercially made straps, too, meant for attaching to fitted and flat sheets to keep them in place. Those straps feature a gripper attachment something like a suspender or garter belt grip.

Could it be too big, with the sheets causing it to buckle? If it is perhaps you can trim it to fit better.

A mattress cover might be the answer, too. I hope that helps.

Our Memory Foam topper that our daughter got us for Christmas a couple years ago, came with a knit Zippered cover, and we don't that problem. It zips on 3 sides so is very easy to remove for laundering, and when you replace it in the cover, you just lay it in, and pull it gently around the edges to make sure it is nice and flat before you zip it up. I don't know whether you can buy just the cover or not. Our topper with cover came from Kohl's. It might also have something to do with the thickness of your topper.
Harlean from Arkansas