Describe the action without using "to be." Instead of writing "I was going to the store," write "I went to the store." In this case, replacing "was" with "went" shortens the sentence and also makes it active instead of passive. Another example is "Mike is a fan of baseball and soccer." By replacing the "to be" form "is," the sentence can be rewritten as "Mike loves baseball and soccer." The sentence is now shorter and more direct.
Choose verbs associated with the message. Since verbs are action words, write a list of all verbs that go along with the subject matter. Don't be afraid to think broadly either. When writing about a football game, "jump," "run," "throw," and "tackle," are all good choices. However, "heartbreaking," "thrilling," and "agonizing," are all words describing emotion, not physical play.
Go through the narrative and circle all of the "to be" verb forms and look for ways to replace "to be" with an active verb. Circle as many forms as possible and then replace them with active verbs. Use a book, newspaper or magazine article, or anything to help practice.