Fitted sheets are designed to stay in place better than flat sheets, but variations in mattress and sheet sizes sometimes make this difficult. Smaller sheets are often pulled too tight to fit deep mattresses, and old sheets lose elasticity. It's frustrating having to refit the bottom sheet after a night's use. Secure the fitted sheet so it stays in place between washings.
Slide a non-stick mat between the mattress and box spring. While this type of pad is usually used to keep rugs from slipping, it works well for sheets. The pad will grab onto the bottom of the sheets when they are tucked into place and help hold them there. The pads need to be wide enough or long enough to cover all four corners. Two pads may be necessary.
Add a mattress topper to the mattress. Choose a topper with tacky, elastic sides that will grip the sheets and keep them from slipping.
Use sheet suspenders to hold fitted sheets in place. Attach the clips under the mattress at each corner and tighten to fit your mattress. Once the clips are in place, the suspenders can stay under the mattress and a new sheet attached. Sheet suspenders are a bit of work because the mattress needs to be lifted, but once the suspenders are in place, the mattress corners only need to be raised slightly to find the clips.