Recut the stems underwater to prevent air bubbles from reducing the water uptake, and use a floral preservative, which you can find at a florist shop, in the vase water. Floral preservative contains a type of sugar for nutrition, an acidifier to lower the pH of the water, and a compound to inhibit the growth of bacteria. Change the water every day if possible. If you recut the stems underwater every few days, your roses will live even longer. If any stem wilts, recut it. Be sure to use clean, sharp pruning shears or some other tool to avoid crushing the stem when you cut.