Do you put balloon balls or something in the water to keep it from sagging in the middle? What do you do to anchor it? A friend of mine says she uses a tarp too and put sand bags around the edge with a ball in the middle.
don't worry about a ball or pillow or anything, it doesn't really help with anything. To anchor the cover I'd use water bags or buckets filled with water. bricks, stones, sand bags etc can be drug into the pool an unlike a water bag or a bucket can be very difficult to get out and can do damage to the pool.