I actually liked the slippery backing on most of my washable bed pads because they worked well to help with repositioning. What if you pinned them to the sheet, or to go with Ahmijoy's idea sew some extra fabric to the sides so you can tuck them in?
Thank you cwillie. I am using a type that works well but one side is slippery and moves around when my husband moves around in bed. Unfortunately that has caused the bed to get wet when the pad ends up at his feet instead of under his butt. If I can find one with a sticky strip on one side, the pad will stay in place until the morning.
Go to your ,local fabric store and ask if they have waterproof sheeting by the yard. This way, you can have it cut long enough so that you can slide it under the sides of the mattress.