Buy filo pastry fresh and use at room temperature. While it is possible to freeze leftover sheets, freezing tends to dry filo sheets out making them prone to crumbling once defrosted. If you do freeze filo, limit freezer time and defrost at room temperature as microwaving to defrost does not work.
Since filo dries out quickly it is important to minimise its contact with air. So it is advisable to get organised with equipment and ingredients before unrolling the filo. Cover the pile of sheets with plastic film or a damp tea towel in between taking individual sheets off when assembling your stack.
Use oil spray or melted, polyunsaturated reduced-fat spread instead of butter and be frugal with the quantity. A light spray or brush is all that is needed between layers. Be a little more generous on the outside layer to develop crispness and a rich golden colour on cooking.