• One quarter cup clear carbonated beverage to one vase water. 
• One cap of hydrogen peroxide every time the water is changed.
• One part lemon lime soda to 3 parts of water.
• Two tablespoons lemon juice, one tablespoon sugar, one quarter teaspoon bleach in one vase of water.
• Two tablespoons of white vinegar, two tablespoons of sugar to one vase of water.
• Use 50% warm water, 50% Sprite or 7UP plus one and a half teaspoons of bleach. 

No flower food? No problem.  "You can put a few drops of Clorox in the water as an alternative,"  The bleach prevents bacteria - one of the main purposes of other additives.
Good old cooking spray helps tropicals - such as bird-of-paradise, ginger, and heliconia - last. "The spray basically seals the pores of the flower and prevents it from drying out, Lightly spritz each bloom once, from about 12 inches away. It will  make them shine, too!
putting copper coin in a flower vase also keeps flower healthy and stay longer