Learn where your legislator is coming from. Do homework on his or her background. Do not ask the typically impossible. Make sure your request is reasonable from a political as well as a substantive viewpoint. Leave your fanaticism at home.
Nothing turns a legislator off faster than a purely emotional argument.
When supporting a specific issue, ask where your legislator stands on it. As a constituent, you have the right to know. 99 percent convinced can translate into a vote against your position. Be polite, but persistent. Remember You do not have to be friends in politics, only allies.