The trick is to always capture SA, it is close enough to other things to keep you involved, also you can capture Centeral America and North Africa without having to hold any more territories than you would need to. Then choose NA or Africa and work on seizing the rest of that, when you complete that if it is late enough in the game you'll win almost every time because Asia, and Europe are impossible to hold, North America is to big to capture in the early game and Austraila as noted before doesn't bring a big enough advantage because it puts you in a poor tactical situation.
EDIT: A good point was made below, this applies only to the standard classic Risk map, in standard Classic Risk.
I think if Australia wasn't overvalued it wouldn't be bad because there is no real non-suicidal way to stop the Aussie snowball once it gets going.
Asia usually lacks a strong player while players focus on their relative home continents, often being sabotaged by competitors or me. That said, I rarely get a chance to play people with Risk experience these days.
We played extensively at school, with lots of politics - so games often lasted weeks (played in-between certain classes) as naturally the weak team up against the strong, ad infinitum.
Every other pair has too many connections and no way to reduce to a chokepoint.