If you're lucky enough to receive a beautiful bouquet of flowers, there's probably a little "care card" inside your envelope telling you how best to keep the bouquet fresh and how best to make it last. If not or if the text is too small to read (I can identify!!) here's a few tips on what to do.
You may have received your bouquet in a water bubble, perhaps in a cardboard vase and it'll be ok in there for a couple of days as there's flower food in the water. After a couple of days, cut the cello off the bottom (over a sink!) and re cut all of the stems on an angle (greater surface area for the flowers to drink) you only need take an inch or two off. Put your new sachet of flower food in fresh clean water in a clean vase and mix, then put your bouquet in the new vase. Keep it away from any heat source (radiators, direct sunlight) and any draughts. If you have roses and they start to wilt, you can re cut them and put the ends of the stems in boiling water (just in a cup) until the bubbles stop coming from the ends. This draws out any air bubbles inside the rose stem that stop it from taking up water. If you're buying a bunch of flowers from anywhere and they've been out of water for any length of time, always re cut the stems. Flowers are clever and will seal their stems once out of water to conserve the moisture they have. If you don't re cut the stems before putting them back in to water, the flowers won't be able to drink it and won't last.