Unwashed containers harbor dirt and germs that can infect flowers and make them rot faster. Clean out containers before using them as vases.
The more water a flower stem can absorb, the longer it will last. If you are cutting a soft stem, cut it at an angle to increase the amount of stem base, leading to greater water absorption. If you have a woody stem, splice it and make cracks in the ends to allow more water to seep in.
Heat will dry out flowers and quickly wilt them. Avoid placing flowers in hot rooms, next to stoves, or anywhere near direct sunlight. They will keep their structure longer.
Stuffy air can be just as bad as heat for flowers. Place cuttings in an area that receives freshly circulated air through vents or other means, such as an open window.
Fully flowered cuttings fade quickly, so cut flowers when the bud is beginning to open. The bud will open fully when the flower is in the vase, and it will last longer.
The more acidic the water is, the slower the flower's metabolism will be and the longer it will take the flower to rot. Add crushed aspirin or another compound to increase acidity. This can change the color of some flowers.
Disinfect the water by adding a few drops of bleach or vodka to the water. The fewer bacteria present, the longer the flower can last.
Stale water is a growing place for algae and bacteria, so at least top the water off now and then, or replace the water entirely. Do not forget to add disinfectant and fertilizer to the new water.
Leaves take up energy and do not always function well after the flower has been cut. If any leaves are below the water level in your vase, cut them off: let only the healthiest leaves remain on the stem.
If the container's mouth is too wide, the flowers will flop and the water can rot the stems more easily. If the vase mouth is too small, the stems can be crushed and quickly die. To preserve the flowers, use a vase with the proper-sized opening for the thickness and number of stems.