The best way to care for your vased flowers is to follow this simple guideline; if you wouldn't drink the water, your flowers shouldn't either. If the water is dirty that is a sign of bacteria in the water and that's bad news. Bacteria not only causes the water to turn cloudy and smell bad, it also blocks the vessels in the stems of the flowers preventing them from being able to drink and shortening their life span. The best practice is to change the water daily. This can easily be done by placing the whole vase and flowers in the sink and running water into the vase until it is clear.
If your flowers came in Oasis Floral Foam, a green sponge like material, topping up the water daily by placing the entire arrangement in the sink until the foam is saturated again is all that is needed.
Please also avoid placing your bouquet in direct sunlight, too close to a heat vent, or close to an outside door where hot or cold drafts may reach your bouquet.