Not only is dry cleaning expensive it's actually terrible for the environment. The main chemical used by dry cleaners is perchloroethylene, which the EPA considers toxic.
It's hazardous for fish, plants and other marine life, and can also affect humans in larger doses.
And most garments that have a dry clean only label can typically be hand washed says EcoSalon, based on an interview they did with Karl Huie of Eco Dry Cleaners.
Also, most clothing companies use a dry clean only label to protect themselves. Since the tag in most garments can't carry several cleaning methods, and the company can be held accountable if something is damaged when the owner follows the listed procedure, the dry clean only label makes it convenient for them.
For clothes that must be professionally cleaned due to their fabric or special work, you should try and use a cleaner that offers either wet cleaning or CO2 cleaning.
With CO2 cleaning, the gas is put under high pressure and converted into a liquid. The nontoxic pressurized liquid works with other cleaning agents and is used like water in a washing machine. Once the cycle is done, the CO2 is reused or released out in the environment.
On the other hand, when you have your clothes professionally wet cleaned, they are laundered in environmentally safe computer-controlled washing machines which spin as slowly as six revolutions a minute and can be personalized for different times, fabrics or fibers.
The EPA has a PDF list of drycleaners that use these alternative methods (PDF). So find a store near you and go green while you clean!