To make sure your flowers look the best all day firstly make sure you find a florist who knows what they are doing. By this I mean, they will have an understanding of what flowers 'hold up' the best for a day out of water, and advise you accordingly.
They will also specially prepare certain flowers ready for a tough day, this may be as simple as wireing and other 'tricks of the trade'.
Once the flowers are in your hands there are a few things you can also do to help.
Keep them in a cool place until needed, maybe in water if natural stems.
Keep out of direct sunlight and away from a heater.
Keep somewhere that they will not be knocked like in the laundry.
While at your wedding and reception, simply be gentle with the flowers (until you need to throw the bouquet).
Place the bouquets on the bridal table or even better have vases of water ready for the bouquets to sit in.