First, plunge the cut flowers into a sinkful of lukewarm water right after cutting, or better yet bring a container of water out into the garden with you.
Put 2 tablespoons of sugar into the bottom of the vase, add water, and stir to dissolve.
Before: flower with leaves intact.
After: run your fingers along either side of the stem and strip off foliage that will be below the water line.
Recut the stem on the diagonal and place into the vase.
Arrange and enjoy!
My flowers last longer than I like to keep them. I just threw out my last group of cut flowers and they were nowhere near being wilted after two weeks.
Another hint: empty the water in the vase after 5 days, trim another inch off the bottom of the stems and refill with water and a Tbsp. of lemon-lime soda. An easy way to do this without having to rearrange the flowers is to grasp the flowers just above the vase line, pull them out and continue holding while you trim the ends with the other hand.