Your senators and representatives pay attention to their mail. The most effective letter is a personal one. It should be concise, informed, and polite. Try to stick to one typewritten page, two pages at most.
Lengthy letters tend to wind up in the bottom of the pile.
If writing longhand, take care to write legibly. It is important to stick with one issue. State your purpose for writing in the first paragraph. Support your position with the rest of the letter. Find out the name and number of the bill you are writing about, then use this information in the letter.