From the About.com website:

You know if you put fresh cut flowers in water it will help keep them from wilting. If you have a packet of cut flower preservative from a florist or the store, it will help the flowers to stay fresh much longer. You can make cut flower preservative youself, however. There are several good recipes, made using common household ingredients.

The floral preservative provides flowers with water and food and contains a disinfectant to prevent bacteria from growing. Making sure your vase is clean will also help. Discard any decaying leaves or flowers, because the freshness of flowers is influenced by the gases and bacteria found on wilted or rotting plant material. Also, don't set your flowers near ripe fruit, because the chemicals from the fruit (such as ethylene) will 'ripen' your flowers.

If you can, keep your flowers in a cool location and out of sun. Minimize air circulation, since it speeds evaporation and can dehydrate your flowers.

Trim the bottom ends of your flowers with a clean, sharp blade before arranging them in the vase containing the floral preservative. Cut the stems at an angle to increase the surface area for water and to prevent the ends from resting flat on the bottom of the container. In all cases, mix the floral preservative using warm water (100-110°F or 38-40°C) because it will move into the stems more effectively than cold water. Clean tap water will work, but if it is very high in salts or fluorides, consider using distilled water instead. Chlorine in tap water is fine, since it acts as a natural disinfectant.

Cut Flower Preservative Recipe #1

2 cups lemon-lime carbonated beverage (e.g., Spriteâ„¢ or 7-Upâ„¢)
1/2 teaspoon household chlorine bleach
2 cups warm water

Cut Flower Preservative Recipe #2
2 tablespoons fresh lemon juice
1 tablespoon sugar
1/2 teaspoon household chlorine bleach
1 quart warm water

Cut Flower Preservative Recipe #3
2 tablespoons white vinegar
2 tablespoons sugar
1/2 teaspoon household chlorine bleach
1 quart warm water

More Tips
Trim away any foliage which would be below the water line. The wet leaves encourage microbial growth that can rot your flowers. Remove any unnecessary leaves because they will accelerate dehydration of the flowers.

Flowers with milky latex-containing sap require special treatment. Examples of these flowers include poinsettia, heliotrope, hollyhock, euphorbia, and poppy. The sap is meant to prevent water loss by the stem, but in a cut flower it keeps the plant from absorbing water. You can prevent this problem by dipping the bottom tips (~1/2 inch) of the stems in boiling water for about 30 seconds or by flashing the tips of the stems with a lighter or other flame.