As someone once said, no one can explain a bill like a constituent. Provide a written statement but don't read it. Briefly reference your main points in your oral presentation and then provide greater explanation and back-up data in your written statement. Use a piece of your letterhead as a cover page to your written testimony, indicating the hearing's topic or subject matter, date, and location, as well as your name and title. Attend prior hearings to see how the hearing process works for that particular committee.
Familiarity is a good thing.
You can identify legislators' key interests and observe their questioning styles, as well as how witnesses behave during their testimony. Brainstorm with others to identify possible questions and how you will answer them. If possible, try to rehearse your answers several times with a colleague or staff person prior to your testimony. Remember your mission and don't get sidetracked. Address one issue at a time.