Cut at least half an inch off your flower stems before putting them in a vase or each time you change out the water.
If your flowers are sitting out of the water on the journey home then typically the ends of the flower stems will dry out and the cells die, making it hard then for the flowers to absorb water. The best thing you can do then is cut a little off the stems before putting them in the water again, the fresh tissue will suck up the water much more efficiently.
You should also trim the stems again when you change the vase water a few days later, remove the tissue that may be breaking down and again expose the fresh tissue to absorb more water.