The following are tips to use when writing to your elected official.
Keep it brief. Letters should never run longer than a page. Congressional aides read many letters on many issues in a day, so it helps if your letter is concise.
Hit your three most important points. For the same reasons as above, you should pick a minimum number of points and expand on those. Members need to remember why they should adopt your way of thinking.
Personalize your letter. Members don't put much stock in letters written by people who are not their constituents. Let them know why this legislation matters in their community and to individuals they represent.
Personalize your relationship. Have you ever voted for your members of Congress? Have you contributed time or money to their campaigns? Do you have any personal or business-related ties to them? If so, tell them. The closer they feel to you, the more powerful your argument is likely to be.
You are an expert. People are often intimidated about contacting their representative's office. Remember that these people work for you and don't know everything about any given issue. A constituent's personal stories can be the most important influence.