Familiarize yourself with different varieties of cut flowers and their individual life spans. At home, re-cut the stems on the diagonal with a sharp knife or flower scissors before placing them in clean water and a clean vase. Remove all lower leaves and thorns below water level, otherwise they will hasten decay and shorten the life span of your flowers. Always use fresh, clean water and refresh at least every other day. Roses should be cut under water, so any air bubbles in the stem have a chance to escape.
Some people have good luck with a can of lemon or lime soda, which also contains sugar.
Keep your flowers in a cool, draft-free area of your home, away from direct sunlight and heat sources, and you will enjoy them for a long time.