Letters to the writer's own senator and representative are especially important. Write in your own words and include thoughts of your own. Your own personal experience is the best evidence. Tell how the legislation affects you. Show as much knowledge as you can, but avoid sending a postcard or form letter.
Handwritten letters are fine if they are legible.
Don't apologize for taking the reader's time. Write briefly, on only one subject at a time. Refer to bills by name and number, if possible. Don't try to become the legislator's pen pal. If you write too often, you become a nuisance.