ZINNIAS: Give them the "wiggle" test. Select a zinnia stem that is mature, and has a stiff neck. Mature zinnias show at least some of a gold ring around the center. Gently rustle the stem at the "neck" below the flower head. If it is firm, this will be a good flower. Don't pick zinnias that still have tender stems, they will break easily.
COSMOS, MARIGOLDS, other small flowers: Select mature flowers here also, ones with sturdy necks. If the neck of the stem is not stiff when you cut it, it will Not get better. Pick quality flowers. Keep the water in your vase very clean! A drop of bleach won't hurt.
Don't only pick the oldest blooms. Younger blooms will last longer. Science Lesson: stare at them until you can see the difference.
Keep the water in the vase very clean, and trim the stems every few days.