In my experience the key for a long-lasting bouquet is to always use a clean vase and water and discard wilted blooms. Here are a few more tips to prolong the life of your cut flowers:
If the water becomes cloudy, it should be completely exchanged. As the water level gets low, replenish it frequently, however best is to change the water every 2 days instead of just topping it off. This is the single most effective thing you can do to keep your flowers looking fresh.
Use plain, lukewarm water for most cut flowers, but use cold water for daffodils, hyacinths and tulips when changing the water.
Flowers keep best when cut with a sharp knife (un-serrated) and put immediately back into water. Always make a cut on a slant, as it exposes more stem surface area.
Keep flowers out of direct sunlight and away from computer, TV or any other electronics that emit heat.
Keep bouquets away from fruit and vegetable, which release a gas that causes flowers to age faster.
Homemade flower food:
To survive, flowers need three ingredients: carbohydrates, biocides, and acidifiers.
The commercial flower food provided works best, however if you rather use a homemade version try this:
1 liter water
2 tablespoons fresh lemon juice
1 tablespoon sugar
1/2 teaspoon household bleach