Florists say cut flowers will use water faster and wilt faster if they are placed in bright sunlight or in drafty locations. The cooler they are kept, the longer they will last, which is why florists keep flowers in a refrigerator. Misting cut flowers also helps to prolong their life.
Flowers often wilt because air bubbles trapped in the stem won't let them absorb water. Trimming stems under water will keep this from happening.
Stems can also get plugged with bacteria. Changing the water each day helps. Before ever placing flowers in a vase, remove the lower leaves to prevent bacterial overgrowth.
If the flowers have been out of water for a while, cut off the bottom inch of the stem for a cleaner cut. Cut at an angle so the stem won't stand flat in the vase.
Sometimes you can revive wilted flowers. Wrap them in newspaper so the stems are straight. Put the stem and paper in a vase of water until the stem stands up again.