Keep the legislator on the subject. State your views clearly. Be specific about desired action. If you disagree on facts, offer to look into it. Don't guess or fudge.
Misinformation is worse than no information.
Write down what the legislator says and promises to do. Visit legislators even when they oppose your position. If possible, leave appropriate printed material which summarizes your points. Don't assume that legislators will disagree with you. Also convey your deep concern.