How do I go about writing to an elected official?

First, make sure to use their correct title, or simply refer to them as "The Honorable______________". Be mindful that, in most cases, a staff member will read your letter and communicate it to your representative. 

Here are a couple of suggestions:
    - If you live in their state or district, so say early in the letter.
    - Be brief. Discuss only one topic per letter.
    - Write in your own words. Be respectful and constructive.
    - Try using the "3 Tells": 
            In the 1st paragraph, tell them why you are writing.
            In the 2nd paragraph, tell them what you are concerned about (why you're opposed, why you agree, etc) Be specific. Quote bill numbers of titles if you know them.
            In the 3rd paragraph, tell them what you told them by asking for a specific action regarding your issue. Ask a question that requires a response (this is always fun). Follow up your Congressman's response (or lack thereof) with another brief letter, regardless of the position they have taken. If your Member agrees with you, send a letter of thanks for his or her stand.

Remember, nothing is more effective than having a pre-established relationship with your Representative. Make time to visit them in their District offices. Volunteer, with your children, in their campaigns and offices. Your time and interest will show these civil servants that you care about legislative issues and the process through which they work.