Try changing the water every other day. leave some water at room temperature. i give my flowers a splash of 7up or sprite when i change the water they love that for some reason. i never use the package that comes with the flowers just sprite or 7up..

Room temperature has a big impact, cut flowers are in fridge trucks so they last longer, if you are somewhere cool leave the flowers outside or near window, avoid heating source, if the flower heads droop, just put them in a tub of cold water (a few ice cubes can help and soak them for 2o or so, it "firms' up the stem again, the cutting at an angle and changing water are of course the main way

Add raw sugar to the water you have them in and cut the stems at an angle rather tham straight across - this helps the sugared water seep into the stems better