1. Keep the vase filled with water containing floral food. Yes, it makes a difference. If a packet doesn't come with the flowers, or when that runs out, you can make your own using a quart of warm (100 degree) water mixed with one tablespoon of sugar and one teaspoon of liquid bleach, stirred until blended. If the water becomes cloudy, replace it.
2. Re-cut the stems by a minimum of one inch before you put the flowers in the vase. Use a sharp knife so you don't crush the cells of the stem, impinging water uptake. Cut on a 45-degree angle to expose more stem surface. Remove any foliage below the water line. Re-cut the stems by half an inch every other day.