Cell membrane fluidity is determined by many things, including the presence of or absence of saturated and or unsaturated fatty acid tails, and because of that the ability of the phospholipids in a cell membrane to tightly pack together or not. I believe though in terms of an additional substance it would have to be cholesterol. These specific lipid molecules are inserted in between the phospholipids preventing phospholipids from tightly packing together thus allowing the cell membrane to have greater fluidity and flexibility.