I think the main reason why Australia has such a high voter turnout is because it's extremely easy to vote. It is compulsory in Australia, but the fine is only $20 (~US$15). Voting always takes place on a Saturday. There are polling booths all over the place, and you can lodge your vote at any booth anywhere in the country. If you know you won't be able to make it on the day then it's quite easy to find a pre-polling station or lodge a postal vote in the few weeks prior to the election.