The main thing that kills cut flowers is bacteria on their stems!

Mouthwash is antibacterial, so it kills bacteria on flower stems in the same way it kills plaque in the mouth, however beware of the blue tint it gives water and minty aroma it will give out if you use too much, so try mint-free mouthwash to avoid that pungent smell.

A copper coin at the bottom of the vase. The copper in the coin is said to act as a natural antibacterial agent, stopping bacteria from forming on the stems.

Clear, fizzy lemonade- 125ml of  per 1 litre of water. Lemonade has two acids (citric from the lemons and carbonic from the bubbles), both of which are antibacterial and help the stems take up water. However, Make sure it's not sugar-free, as sugar provides valuable nutrients.

A teaspoon of vodka per litre of water. Alcohol is a known antibacterial agent, so it kills bacteria on the stems. It's also said to stop the production of ethylene - a gas emitted by plants that helps them to mature.

One crushed aspirin, dissolved in the water, per litre . Aspirin contains the naturally-occurring salicylic acid, which discourages bacteria and helps the flow of water through the flower. 

Bleach¼ tsp of bleach per litre of water. It seems so wrong to put delicate flowers in a vase with a germ-killer, but just 1/4 tsp diluted in water workd. As long as it's not too strong, bleach will kill bacteria on the stems. It  also sterilises the water and stops it from going cloudy- which so often happens with flowers after a few days in a vase.