For cut flowers that last for the longest possible time, start by harvesting early in the day, after the dew has evaporated, but certainly while the stems are fully turgid (rich in moisture). If the garden is particularly dry, it may be even worthwhile watering the day before.
Sugars too will be most highly concentrated on morning-cut blooms and that can add days to their vase life.
Bring a pail of warm water to the garden with you as you harvest. Each time you cut a stem, immediately plunge the cut end into the water. This will allow for better hydration. Why warm water? Because flower stems absorb warm water better than cold.
Early harvesting for more flower power? Why not?