Be polite and never threaten. Use your name and address on both the envelope and the letter. This helps staff in replying and identifies you as a constituent. Write to thank your legislators when they take an action you agree with. It's surprising how few letters of thanks are received on Capitol Hill.
Telegrams can be useful when important votes are coming up.
They are most effective if you have already sent a letter. Telegrams should be brief, stating your position on the upcoming bill, and should include the bill number. They can be sent only to elected officials and are delivered to the White House or Congress the same day they are sent. Visit a nursing home in your neighborhood and volunteer for a variety of activities. Bring in a bunch of Dunkin Donuts coffee, pass it around.