There is flower food (comes in a small packet usually with the flowers) that you put in the water. I don't recall the ingredients.
What you need to do when you buy flowers is cut the bottom stem at a 45 degree angle while the stem is still submerged in water. That prevents air from penetrating the cut right away and should lengthen the life of the flower an extra couple days. Let the freshly cut flowers sit in water for at least 5 minutes before transfering them to a vase, and then add the "food" to the vase water.
(mom owns a flower shop, and it's amazing what information you retain even when you aren't paying attention)