Provide the bill number when possible. Leave a brief message for the legislator that articulates your position. Ask the legislator's position on the issue. If you are a resident of the representative's district be sure to let the staff person know. Legislators tend to be more responsive to calls from constituents in their districts.
Telephone calls are also useful to follow up on previous communications.
Remember, don't assume a single communication will do the job of getting the legislator's vote.