S.G.
I worked in a florist for approx 3 years with a wonderful MAN who had been in the business for over 50 years. He was extremely conscious about NOT selling "old" flowers. We had daily deliverys and once the flowers had been in our shop for 7 days, he would no longer use them in arrangements to sell to the public. I became very adept at keeping flowers fresh. My dear neighbor used to visit shut ins and I would use these "old" flowers for the shut-ins.
TIP #1: The cut. Most flowers get their nutrients from the stem. Every 3-4 days cut the stems in a 90 degree angle using a knife. (Always make sure you are cutting away from your body.) Some flowers (especially some brand of roses) have extremely tough stems. If you must, you can use a pair of shears, but ... sometimes this crushes the stem and the flower cannot get the nutrients. Put the freshly cut flowers in fresh water. You can purchase packets of preservatives from your local grocery florists very inexpensively or ... a craft store. There are many different brands but for home use most of them are the same. (My employer insisted on one professional brand that you cannot purchase OTC.) If you cannot find the preservative, use plain cold water. If the water is too cold, it will cause the flowers not to fully bloom.
TIP #2: Greenery. "Leather-leaf" which is the greenery most florists get their nutrients from their leaves - not stems. Spray the greenery with a spray bottle. Try not to spray the fresh flowers as it will damage them.
TIP #3: Carnations and, believe it or not, Roses last the longest. You can dip a small (q-tip size wad) cotton ball wad into preservative or just plain COLD water and using a toothpick shove down in the middle of the carnation. This will preserve them for a month or more. Always remove the yellow stems from lillies the minute you get them. (Some florists leave them in and the lillies will shrivel very quickly.) If the lillies are still closed, remove them when they open.
As you cut the flowers they obviously will get shorter so sometimes you have to use a shorter vase. With large arrangements, it is sometimes easier after the first couple cuts to arrange into 2 or 3 separate arrangements. HAVE FUN!!!
There are many more tips, but ... this answer is long enough and I don't know what kind of flowers are in your arrangement.