Re-cut the stems of your flowers with clean sharp secateurs, flower snips, or scissors and place them in fresh cool water right away. Ensure that no foliage remains on stems below the water line, as this can foul the water. Refresh the water every 2-3 days. You can also re-cut the stems any time you change the water to extend flower life further.

Keeping your flowers in a relatively cool place, and out of direct sunlight, will help them last the longest, so avoid south-facing windowsills and radiators.