Changing the water every day will help extend the life of the flowers.

Regular trimming of the stems will help the flowers to maintain a good water level. Cut about 1/2 inch off diagonally for best effect.

Keep the flowers in a cool place. Keep them out of direct sunlight if it is very hot, and not directly under air conditioning units.

Adding a bit of sugar to the water is supposed to add to the endurance of the stem.

Sun is good, lots of it isn't. Daffs love it

If you want the flowers to bloom quickly, add a Dispirin tablet to the water. I haven't tried this myself, but I read it on the internets so surely it must be true :-)