In the evening when you are ready to call it a night, try keeping flowers in your refrigerator to help preserve their freshness. Flowers survive best in colder temperatures between 40-50 degrees. Just make sure your refrigerator is not too cold to where you freeze your flowers.
Remove all leaves from the part of the stem that is submerged under water. You can clean the stems by lightly brushing them with a vegetable brush. This will help the flowers drink as well as keep the water clearer.
Change the water and mist the flowers with water everyday. Keep the flowers out of direct sun and away from heat and drafts. This includes ceiling fans. These things will cause your flowers to wilt quicker.
Lastly, remove any dying flowers immediately. Dying flowers emit ethylene gas that will cause the other, healthy flowers to wilt.