Legislators are not likely to respond well to blame or anger. An angry letter can do your cause a disservice. Identify as precisely as possible, including the bill number if available, the issue that concerns you, state your view on that issue, and provide specific statistical and anecdotal evidence supporting your view. Acknowledge counter-arguments and evidence. Legislators want to have reliable evidence on which to base their decisions.
Legislators receive a great deal of mail.
A concise letter of one page is the best vehicle for your message. If you are interested in more than one issue before the legislature, write one letter per issue. Say thank you. Legislators rarely receive thanks for their efforts. If you are aware that a legislator has been supportive in the past, let them know that you appreciate their efforts.