To create a pretty bouquet:
First, choose roses which have already been formed and are beginning to open, with a few petals starting to unfurl. They will have a greater life span than unopened or already-opened flowers.
For your flowers to last, once you bring them home from a flower shop (or select from a rose bush in your garden),
cut an end of a stem using a sharp knife or shears and put it in cold water immediately after cutting, so that no air enters the stem.
When your flowers start to fade, plunge the ends of the stems into boiling water, refresh the water in you vase and then put the flowers back.*
Enjoy your roses and let me know how the technique works.
*from the book by Laura Fonty, Truces & Astuces (Tips & Tricks), Editions France Loisirs, 1985