To care for a mixed flower arrangement and prolong its freshness, it's important to follow a few essential steps. First, cut the stems of all the flowers at an angle before placing them in a vase with clean water. Change the water every other day and add a floral preservative if possible. Keep the flowers in a cool place, away from direct sunlight and heat sources. Since mixed flowers can have different care needs, it's important to check each type of flower and make sure they all receive the proper treatment.