The very first thing you should do is cut the stems before you put them in a vase. Take each flower, one at a time, hold it under water and cut the bottom of the stem diagonally. Also, Steve says it's very important to change the water every day. If you want the flowers to open up faster, make the water luke warm. If you didn't get a preservative packet with your flowers, put a tablespoon of sugar in the water.
Sugar is a nutrient for flowers.
But again, make sure you change the water every day. And here's a tip for droopy roses.