The best things you can do to keep your flowers fresh and lasting the longest are:

1. Change the water in the vase everyday. Bacteria in the water is what make flowers go down faster, so if you completely change the water everyday this will keep bacteria from forming.
2. Re-cut the stems of the flowers every other day. This allows the flowers to drink better, and also removes bacteria on the ends of the stems. Cut about one-half inch off when you re-cut.
3. Never put your flowers in a sunny window or near a heat vent or on anything that emits heat. By keeping your flowers in a cooler location they will last longer.

Remember, fresh flowers drink more water. They can drink a vase full of water in one day. This is why changing the water every day also helps ensure there is plenty of water in the vase.

If you do these things, you should never have to add floral preservatives or anything else to your water. Floral preservatives work well for some flowers, but will actually harm others. Most garden flowers do not like floral preservatives.