You should never pick flowers in the middle of the day when the sun is at it's hottest.
The heat of the sun lowers the water content in the stems and the flowers will not last nearly as long.  If it has been raining and the flowers are wet, shake them gently to remove the excess water. Too much water will often damage flowers - especially delicately petaled flowers.

When to Pick Flowers
Most flowers should be picked when they are in bud or half open.  You will then have the pleasure of seeing them slowly open up.  The colour of the petals should be starting to show.  If picked too tightly in bud, they may never open.  This is especially true of tulips and roses.  The green pointed sepals around the base of the rose should be starting to turn downwards.  Irises and daffodils should be half opened.  Gladioli should be picked when the bottom three or four florets are open and the top florets are still in bud.  Carnations, dahlias, marigolds, hydrangeas, camellias, gerberas and chrysanthemums should be picked when they are fully opened.

Fill a plastic bucket a third to half way with warm water. Warm water should be used as flowers will take up warm water more readily than cold. Its preferable to add preservative to the water. (The use of preservatives is fully explained further on). It is advisable to use plastic and not metal buckets as the chemicals in the metal can affect flowers and can neutralize the effect of the preservative. Flowers only drink through the ends of the stems and not through the sides of the stems, and for this reason buckets should not be filled right up to the top with water, as foliage left on stems below the water line will rot and pollute the water. 
This will cause bacteria and the flowers will die more quickly.
The foliage of marigolds, chrysanthemums, stock and daisies send off a particularly strong odour when left standing under water over a period of time.