When you receive your flowers in a box, keep them in a cool place until you are ready to place them in a vase.
Foliage under the water decays faster, thus shortening the flower's life, so remove any foliage that will be under water when you arrange them.
Cut an inch off the base of each stem using a sharp knife and place them in cold water. 

If enclosed (some flower packs do not require this) add the floral preservative to the water according to the preservatives instructions. 

Check the arrangement daily and replenish the water as needed.

Keep the vase filled to the top to ensure all the Flowers have an ample water supply.
Keep your flowers out of direct sunlight and away from heat sources and drafts.

 
Flowers love a cool well lit area, but not in direct sunlight. Avoid placing the flowers near to fruit. 

FLOWER DRYING:
After the flowers and roses are no longer fresh you can hang them upside down in a dark dry area and dry them. These can be used for a home made dry flower arrangement, designed by you. Or why not make a POTPOURRI with dry rose petals by mixing them with perfume after they are dry.