Use active voice as much as possible. “The boy threw the ball”, not “The ball was thrown by the boy.” Note it also eliminates a preposition. This is another way that tighter writing is better writing.  There are exceptions, but use them consciously. For example, a statement such as “The board approved the new proposal” is often simply stated “The new proposal was approved.” Watch for needing a prepositional phrase following the verb, in which case you may be slipping into passive voice.