The following tips will help to make any contact you attempt more effective:
Make sure you include your name and address - politicians are likely to pay most attention to people who live in their electorate.
No longer than about one page (or equivalent for email) and keep to one issue only. Be as concise as possible.
Original letters are more effective than form letters/emails sent by dozens of people. Even if your writing skills are not the best, a letter written in your own words will carry much more weight than regurgitating what some else said.
Include a subject line. If it is about a specific piece of legislation (an Act) or a proposed law (a Bill), state the full name of the Act or Bill in the subject line, or at least in the first paragraph.
Start with a clear statement of purpose for example: "I am writing to ask you to support / oppose ..."
Sticking to the one issue, choose three important points that are most likely to be persuasive in gaining support for your position and flesh them out. This is more effective than attempting to address numerous points in a letter.
Ideally something concrete for example, in relation to a proposed law (a Bill), ask them to raise the matter in their party room and seek to have their party oppose the Bill.
While the response will usually be a form letter, written and authorised by their political party, you will know you have had an impact on their office. Party politics in Australia are such that few elected politicians are likely to tell you whether or not they personally share your views/position. However, a well-written letter can be instrumental in prompting them to take action behind the public scenes to inform and potentially change their political party's position.
When possible, include a personal story and/or information on how the issue affects you, your family, your business, or people around you. This can help your representative understand your position and can be very persuasive as he/she forms a position on an issue. The more personal your letter, the more impact it is likely to have.
Personalise the relationship. If you have ever voted for the representative, or contributed time or money to their election campaign, or have met them, etc, say so. The closer your representative feels to you, the more effective your letter is likely to be.
Be courteous, but don't be afraid to take a firm position. While your representative's job is to represent you, remember that politicians and their staff are people too. Threats, hostile remarks and rude/offensive language are among the fastest ways to alienate people who could otherwise decide to support your position in light of rational and reasoned argument. Avoid creating enemies.
Thanks is as important as criticism. Politicians and political parties need to be able to tell the 'other side' that they have been inundated with calls and letters supporting their position. Write thank you letters to politicians/parties that you know support your position. This will encourage them to stand firm on their position rather than backing down, which has often happened during the passage of proposed laws through Australian parliaments.
Keep the irritation factor low. Avoid accusing/criticising the wrong politicians/party. Politicians, like anyone else, may become irritated when accused of holding views they do not. If you are not sure of the views of the person or political party you are contacting, either research the matter, ask them, or just inform them of your views and why they should support same.