The only important criterion in maintaining fizz is the maximum potential volume above the liquid. Since different gases exert partial pressures independently, the partial pressure of carbon dioxide produced in the top of a half-empty bottle is independent of whether or not there is also air in that space. Thus assuming one replaces the cap securely - thus containing the volume, it makes no difference whether one squeezes the flexible bottle to expel gas above the liquid first, or not. In either case the maximum potential available volume is the same (because in the squeezed case the bottle will expand back to its original shape). The only way to better preserve fizz is to _permanently_ reduce the available volume e.g. by 1) dropping stones etc. into the liquid before tightening the cap, or 2) squeezing the bottle, replacing the cap, _and_ maintaining pressure to keep the bottle crushed or 3) filling the volume above the un-squeezed bottle with pure carbon dioxide before resealing.