Wash Your Vases Thoroughly Before Using Them Again
When it comes time to throw out your last arrangement, make sure to very thoroughly clean the vase/container with hot soapy water or, even better run it through a cycle in your dishwasher.
Bacteria from older flower arrangements build up in dirty vases and will not go away if the vase dries out. Once you add water again, the vase will be full of bacteria that will start to attack your new bouquet. If you give your flowers a clean, fresh place free of bacteria they will last a lot longer.