Don't hesitate to write for fear of imposing on them. If you have something to say that you think should be called to their attention, do so. The mail gets top priority with most elected officials because every letter represents a potential vote. Most representatives, in the interest of retaining their seats, most want to keep the voters happy. Do write legibly.
Handwritten letters are fine if they are readable.
Do use your own words and your own stationary. Do be sure to include your address and sign your name legibly. If your name could be either masculine or feminine, identify your sex. If you have family, business, or political connections related to the issue, explain. Do be courteous and reasonable.