Keep it short. A longer letter will be set aside for reading/answering as time allows. You do not want to be set aside. A well worded short letter is just as effective, sometimes even more so than an epistle.
Keep it neat. A typed letter is obviously easier to read, but not always practical for everyone. If you do not have access to a computer or typewriter, simply hand write the letter as neatly as possible.
Personalize your letter. Form letters do not carry the same weight as individualized correspondence. When writing, identify yourself or your organization. Mention whether you are a constituent or if you have another connection to the legislator's district.
Be courteous. A belligerent letter will not win any points with a representative or his staff.
Armed with these suggestions, you should be set to write an effective letter with the power to convince your representative to introduce new legislation or support your view.
Dear [Senator/Representative/Delegate ________________ ]:
[Include a short paragraph about yourself]
Federal Marriage Amendment
Marriage in the United States shall consist only of the union of a man and a woman. Neither this Constitution or the constitution of any State, nor state or federal law shall be construed to require that marital status or the legal incidents thereof be conferred upon unmarried couples or groups.
[Copy and Paste this sample, then edit it to personalize yourself.]