There are legitimate uses for the passive voice:
           “This absurd regulation was of course written by a committee.”
But it’s true that you can make your prose more lively and readable by using the active voice much more often.
         “The victim was attacked by three men in ski masks” isn’t nearly as striking as “Three men in ski masks attacked the victim.”
The passive voice is often used to avoid taking responsibility for an action:
         “My term paper was accidentally deleted” avoids stating the truth: “I accidentally deleted my term paper.”
Over-use of passive constructions is irritating, though not necessarily erroneous. But it does lead to real clumsiness when passive constructions get piled on top of each other:
           “No exception in the no-pets rule was sought to be created so that angora rabbits could be raised in the apartment” can be made clearer by shifting to the active voice: “The landlord refused to make an exception to the no-pets rule to allow Eliza to raise angora rabbits in the apartment.”
Visit Paul Brians’ Common Errors in English Usage Web site: http://www.wsu.edu/~brians/errors/errors.html

EDITOR’S NOTE: A sentence that does not emphasize the important actor in it is not necessarily passive. The passive voice is constructed by using the verb “be” or “get” with what is known as the past participle form of the verb, as in “Hamburgers are served at Wendy’s” or “Elaine’s flight got delayed.” It is surprising how many people, even professional educators and writers, do not understand the grammatical structure of the passive voice. Geoff Pullum of Language Log discusses one recent example here [Drones and passivity]:  http://languagelog.ldc.upenn.edu/nll/?p=3549.