There are some cut flowers that have special needs.
Carnations, for example, are really serviceable flowers to buy, as the last a long time in a vase. To prolong their life, cut the stems at an angle between two joints, and cut underwater to prevent air bubble from forming in the stems.
Another long-lasting flower is Alstroemeria, and it benefits from having its stems split and then being conditioned in lukewarm water that is high enough to reach just below its flower heads.
Cut dahlia flowers as soon as they open, and hold the ends of the stems in boiling water for 30 seconds or singe the ends with a candle flame before plunging them into warm water. Shield leaves from steam and, of course, remove all leave that will be below the vase's waterline.
Gerber daisies, either from the garden or from a store, should have their stems dipped in boiling water before being placed in cool water that comes to just below their flower heads.
Delphiniums are best cut when about half of the florets are open, and then they should be conditioned in deep, lukewarm water before they're arranged.
Cut gladiolus when the next-to-lowest floret is ready to open, split the stems and place in water overnight. Use warm water.
Remember: split or crush woody stems and sear milky stems.