I once heard someone suggest that it would be a good idea to give a bit of vodka to tulips to help them stand up a little more straight. Well I tried it. My own feeling in this area is that you are probably better off using preservatives that you can pick up from your local florist, as they have been developed for optimal results. Changing the water every few days is helpful. You see, petals, and leaves fall into water that sits for a few days, and the water gradually does get a bit dirty.
Bacteria build up in the water makes it harder for the flowers to take up water.
Only for a day or so if you have a special use. Fruit puts out an invisible gas called ethylene. In a closed area such as a refrigerator it can be bad for your flowers. I heard that I should cut the stems of my flowers before placing them into water. Floral clippers, a sharp knife or scissors will all work.