First, recut the stems at least 1 inch shorter than when you got them. Use a sharp knife and cut at a 45-degree angle. Immediately submerge the stems in warm water. Even better, cut the stems while they are under water.
Be sure and strip off any leaves that would otherwise be immersed in the vase water. Add a packet of flower preservative to the water. This often comes with the bouquet. If you do not have the floral preservative packets, change the water daily in the vase.