Change the water in your vase every few days and add flower food. If you don’t have flower food, add a couple drops of bleach to prevent microbe growth and a few teaspoons of sugar to feed the flowers.  Cut off a quarter inch from the bottom of the flower stem when you change the water. This is essentially your flower’s filter, and as it gets clogged over the course of a few days sitting in water, a quick trim can refresh the filter and let your flowers take a full drink once again.