What do you do with 80 roses or for that matter a few stems of any cut flower?
It's pretty much the same for florist and garden cut flowers. Just a few always and most cut bouquets should last at least a week and sometimes weeks.
Make sure the container is sparkling clean. Wash the residue out of a used vase with hot water and a good detergent. Rinse. I usually run mine through the dishwasher after hand washing the residue off. Aside from making the vase pretty, it's essential to remove any and all bacteria.
The little packets of professional grade preservatives have a mixture that will help the flower take up nutrients, water and prevent decay. Mix into the warm water.
Remove all foliage that will sit beneath the water line. Wet foliage will quickly rot and ruin the bouquet.
Cut off the end of the stem and insert immediately into the water. Any stem that has been cut for even a few minutes will have formed a barrier over the end. The new cut allows water to be taken up.
Don't sit the vase in bright direct light or where it will be hit by a cold burst of air (near outside doors) and not on or near heat sources.
Remove flowers as the wilt. When a good portion of them have been removed, take the flowers that are left and trim for a smaller vase.
According to the University of Illinois garden site, doing the above will prolong the life of flowers and foliage.