When cutting hydrangeas, cut them just as blooms fully develop. Cut your flowers in the early morning, before the sun comes up to evaporate some of their moisture. Cutting at diagonal will allow the stem to take in the most amount of water, some people will even cut slits or fray the ends of the stems a little. Place your freshly cut flowers in a bucket of cool water to soak for an hour or two before arranging your final product.
Once you have your flowers, use a commercial floral preservative to get the best results. This will feed your flowers, maintain a constant pH, and will serve as an anti-microbial to prevent premature decay. You should be able to find this at a local nursery. Keep in mind that many gardeners and florists complain that hydrangeas wilt faster than other cut flowers and may require a little extra planning.