When you wake up in the morning and find your pillows out of their pillow cases, there are several things that you can do to prevent this annoying little occurrence from happening.
Sew Velcro Strips
One method is to sew some Velcro strips at the opening of the case in order to enclose the pillow and prevent it from falling out. If this is something you cannot do for one reason or another, try the following method.
Buy zip-on covers
This method is almost similar to the one above; buy zip-on covers so you can totally enclose your pillows and prevent them from falling out while you sleep. You can also try getting covers that fit more snugly for your pillows if your current ones are too big. This could be the cause of your pillows sliding out of their respective pillow cases.
If none of the above methods work for you, you can get a couple of safety pins and use them to pin together the top and bottom flaps on the open end of your pillow case. This should stop your pillow from coming out of pillowcase while you sleep.