As their name implies, fitted sheets are designed to fit over a mattress of the same size. In reality, not all fitted sheets stay in place as they should: A corner of the sheet may pop off the mattress when you move slightly in bed, or when you're making it up and attempting to fit the opposite corner in place. No matter how frustrating those ill-fitting sheets, specialty clips or straps can help keep them in place.
Fitted Sheet Frustration
In some cases, a fitted sheet may not fit the mattress even when it is brand new. A plush mattress such as a pillowtop is significantly taller than a standard mattress, so a sheet designed to fit a standard mattress cannot be pulled all the way down to the bottom of each corner no matter how hard you try. To prevent this sheet-purchase faux pas, measure all the dimensions of the mattress, including the thickness or height of the mattress; then check the measurements on the sheet packaging before buying another set of sheets to ensure they'll fit.
If the fitted sheet is large enough to fit the bed but still won't stay put, a specialty item similar to suspenders keeps all four corners in place without affecting your sleep. Each elastic strap clips onto one corner of the fitted sheet, then attaches to the corner diagonal from it, all between the mattress and box spring. The straps stay under the mattress when it's time to change the sheets; simply unclip the current sheet. Then clip the clean sheet in place.
Tiny Strap Solution
Small elastic straps with clips at each end also help the sheets stay in place. Fit the sheet onto the mattress as you normally would; then clip one end of the strap several inches from one corner of the mattress, such as along the side near the foot of the bed. Slide the strap under the mattress and out the adjacent side, also near the corner, clipping it to the sheet in that area. Repeat the process with the remaining three corners.
Add your own elastic straps, permanently attached to the fitted sheet, to ensure the bedding stays on the mattress. Use 1/2-inch-wide elastic; then sew it so it spans each corner on the diagonal, several inches away from the actual corner on each side, along the open end of the sheet. When fitting the sheet to the bed, stretch the elastic and slide it under the corner of the mattress to keep it secure.