I've posted some links below that may be helpful to you. One of them suggests handwriting the letter and sending it by postage rather than merely emailing it. This seems to be good advice.
I would also suggest some of the following tips:
- Refer to specific legislation you either endorse or oppose.
- Be as clear and specific as possible about your reasons for endorsing or opposing the legislation.
- If you have no specific legislation in mind but would like to suggest possibilities for future legislation, try to be practical. Make suggestions that are affordable and likely to win broad support.
- Show that you are familiar with feasible, practical solutions to any problem you want Congress to address.
- If you have access to some particular source of information that you think may be helpful, provide references and/or web links.
- Make sure that someone besides you proofreads your letter before sending it. Preferably this person should know what constitutes clear writing and should know the rules of grammar and punctuation.
- Consider this possible model letter: http://www.calvert.com/climate-action-congress.html
- Show that you are reasonable and are aware of the real-world problems involved in achieving the goals you support.
- Indicate ways in which the goals you support can be useful in supporting other goals, such as job creation, reducing the budget deficit, reducing the size of government, etc.
- Here is another link worth looking at, since it contains sample letters: http://www.efficiencyfirst.org/policy/
- Indicate any special expertise you may possess.
- Consider reinforcing your letter with phone calls to the relevant offices, especially local offices.
- Consult this web site for highly specific information about whom to contact: http://www.contactingthecongress.org/
- Remember the words of former Representative Lee Hamilton in his book Strengthening Congress:
Members of Congress typically give high priority to responding to letters from constituents, and you will almost certainly receive a response.