Check your flowers daily
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. When you completely change the water, you should also thoroughly clean the container. Add fresh water only. Although commercial fresh flower food contains the correct ingredients to properly nourish the flowers, and keep bacteria at bay (which can block the stems and prevent them from absorbing water) Improperly mixed flower food does more harm then good. Cleaning the container and adding fresh tap water (avoid softened whenever possible) every few day will keep your flowers the freshest.
Re-cut their stems
Every few days, remove the flowers from the design and re-cut the stems. Remove 3/4" to one inch from the bottom of the stem and be sure to cut the stem at an angle to allow the flower the best chance to take up water. Use a sharp knife or sharp floral cutter or scissor. Avoid smashing or piercing the stems, as this can destroy the water vessels in the stem and prevent water absorption. Remove any spent or damaged blooms or foliage that falls below the waterline in the vase.
To avoid disturbing the stem placements in a vase arrangement, tie the stems with twine just above the vase’s edge before taking them out of their vase. Hold the tied arrangement at the twine-bound point to re-cut the stems. Then place them back into the clean vase with the fresh water.
Change their water
Be sure to clean the vase before refilling it with room-temperature water. Be sure no debris is floating in the water (such as leaves and stem parts) as this could promote the growth of bacteria which can shorten a flower's life.
Display them in a cool spot
Most flowers prefer temperatures between 65 to 72 degrees Fahrenheit (18 to 22 degrees Celsius) and are best displayed away from direct sunlight, heating or cooling vents, directly under ceiling fans, or near televisions or radiators, which give off heat and can cause flowers to dehydrate. Avoid placing fresh flowers near ripening fruit, which releases tiny amounts of ethylene gas that can age them prematurely.