Choosing the active voice—instead of the passive voice—is your most important step to a powerful speech. The active voice relies on verbs. "The boy ran" is more powerful than "the boy was seen running." The active voice has a clear subject, and in speeches that subject is usually you. In an active sentence, the subject is "doing" something (for example, The boss vetoed John's idea). In a passive sentence, the subject is receiving the action—something is happening to the subject (for example, John's idea was vetoed by the boss).
Paint a picture in your mind of what you want to say, and choose the brighter, bolder image. Think of it as a movie. Here are two scenes: In the first, Tim comes into a room. He's angry. He wants to complain to someone. He takes out his pen and begins to write. In the second version, the camera pans slowly across the room and settles on a blank piece of white paper. We see someone's hand holding a pen, and words begin to appear on the page. Scene two can be effective, if that's what you're going for, but it you want the audience to get your point immediately, scene one is going to be much more powerful.
The active voice also gives the subject responsibility; the passive voice takes it away. If I use the passive voice and say, "The dishes must be washed before we leave," who is responsible for washing them? It's hard to tell. If I use the active voice and say, "You must wash the dishes before we leave," there is no question that "you" must take the action. You take responsibility by saying "I saw," or "I believe." The passive and impersonal "It has been seen that," rather than "I saw," may remove you from the line of fire, but it makes for boring speeches. If Caesar had spoken that way, his powerful Veni, Vidi, Vici" ("I came, I saw, I conquered") would have been: "The place was arrived at, was observed, and was duly overtaken."
One way to keep your language active is to eliminate verbs that end in "-ing." "I run" is stronger than "I am running." A title such as "How to Run a Meeting" is stronger than "Running a Meeting."
The active voice takes the more direct route to your destination. That doesn't mean you never want to use the passive voice. There may be times when you want to slow the action down, to paint a picture, or go get a different rhythm.