Use a real life story. This is the most important rule of all. Testify about what you really know - your personal experiences or the experiences of one of your members. Legislators respond to human interest stories - not just facts and figures. Keep it simple and avoid jargon.
Legislators deal with literally thousands of bills, most outside their area of expertise.
On any given day, they may be confronted with 15 to 20 different issues. So if you want to have any chance of holding their attention and persuade them, keep your message simple and avoid using jargon. Mention if you are a constituent of one of the members of the committee. As someone once said, no one can explain a bill like a constituent. Provide a written statement but don't read it.