Go to Linens & Things or Bed Bath & Beyond or anyplace else that sells bedding and look for “sheet clips” or “sheet suspenders”. These are clips with elastic bands attached –you clip them the lower edges of the mattress cover and anchor it beneath the mattress.
You could stitch elastic on the diagonal at the corners maybe 6 to 8 inches in and slip those under the matress.
Go to Linens & Things or Bed Bath & Beyond or anyplace else that sells bedding and look for “sheet clips” or “sheet suspenders”. These are clips with elastic bands attached –you clip them the lower edges of the mattress cover and anchor it beneath the mattress.
You could get some Velcro material and either sew ar attach the pieces with safety pins for removal later. Some even has sticky side to install.
You can get Sheet Suspenders or Sheet Straps …
A sheet suspender is a of elastic banding that looks like men’s pants suspenders. It is attached under the mattress and holds the sheet in place. See image here:
http://www.bedbathandbeyond.com/product.asp?order_num=-1&sku=10474167
Sheet Straps are short lengths of elastic banding which attach at the corners to hold the sheet or comforter snugly in place. See image here:
http://www.bedbathandbeyond.com/product.asp?order_num=-1&SKU=10220599&RN=404
There are other items at Bed Bath and Beyond which may be better suited to your needs, but these two are a good place to start.