Don't ask the impossible. Don't use threats, but be courteous and respectful in all communications. If a bill deals with a problem, but seems to represent the wrong solution, propose constructive alternatives. Take a position. Ask your lawmakers to state their positions on the issues in their replies.
Personal communications are more effective.
Time your letter when the bill is in subcommittee or committee, before it is approved and sent to the floor for a final vote. Write when the lawmaker does something that deserves approval. A word of appreciation will create a more favorable light for the next communication. On occasion, if you know your lawmaker well enough, you may call to express your views. Jot down the ideas you wish to convey to your lawmaker ahead of time.