The most important thing is to cut the stems on the flowers before you put them in the water...each day change the water and try to at least every other day cut the stems a little...some people say to add an aspirin?????????
I don't there's anything that will keep the water clear for 2 weeks unless you change it out everyday. It also helps to strip all the leaves at the bottom. Make sure there aren't any leaves submerged in the water.