There two real important requirements which you must stick to: a cool place is needed and always wet soil. After blooming period the best you can do is to cut the stems half of their lenght.
There many kinds of this beautiful plant . For example, Hydrangea macrophylla (the most popular hydrangeas grown in home gardens and landscapes) - blooms in pink, white, violet and blue colors. The pink sorts can be "painted" blue by adding a special patented substance in the soil, but only before starting the blooming process.
So here's a resume of the secrets of growing and taking care for a hydrangea at home or in your garden:
Temperature: coldly place , minimal temperature through the winter 7 degrees.
Light: bright, but not direct. 
Watering: Through Spring to Autum soil must be always wet. Don't do the watering with chalk water.Humidity: from time to time spray the leaves with water (once a week. )
Cares after blooming:
Move it in a bigger pot (one size), and continue watering like before, you can also supply it with special hydrangea mixture, fertilizing twice per Summer will be the best you can do. In the hot season you can move it to an outdoor place ( a terrace or a garden ); In Winter - a cool room, less watering, and in the middle of Winter move to a warmer and a brighter room and start watering more.
Some other advices I could give you is tobe very careful when you choose the right location of the flower, to be strict with watering as we said. If plant's leaves turn yellow , it is most probably because lack of iron. Hydrangeas don't have to be pruned back, only if they are old enough ( 5 years at least ), prune dead blooms and only " for the health of the plant" reasons. The best at all is to plant the flower to a place where it won't be have to be pruned.Check out my collection of hydrangeas. There are plants and cutflowers into wedding arrangements and other bouquets , hope you enojoy it :)
