When you cut a stem underwater, a drop of water clings to the cut surface as you move the flower to the vase, preventing air from becoming trapped inside the stem. Trapped air will cause the flower to wilt prematurely. Display your arrangement in a cool draft-free location, away from harsh sunlight and heater vents. Don't set flowers on top of a television or other heat-producing appliance. Top off the water daily, and replace it completely after several days.
Flowers like to be misted too.
Remove faded blooms, and when the arrangement dwindles down to just a few blooms, move them to a smaller vase.