Keep the vase, container or liner filled with fresh water. Even if the container has floral foam, you should keep the water level high to promote long lasting flowers. The container's water has a fresh flower food added to it. If you completely change the water, you should also thoroughly clean the container. Add fresh water mixed with a commercial fresh flower food. These flower foods may be supplied by the florist who delivered your order and they contain the correct ingredients to properly nourish the flowers, keep bacteria at bay (which can block the stems and prevent them from absorbing water), encourage buds to open, and lengthen the life of the bouquet. It's one of the best - and easiest - ways to extend the life of your flowers, but be sure to follow the directions on the package correctly. Improperly mixed flower food can actually do more harm than good.